spec: Fix grammatical error

We have a several instances of "does not exists" in the spec, probably
caused by copy-pasting.

To fix it and prevent new cases from appearing, this patch replaces it
with the proper "does not exist".

Author: Akrog

spec.md

@@ -897,8 +897,8 @@ The CO MUST implement the specified error recovery behavior when it encounters t
 
 | Condition | gRPC Code | Description | Recovery Behavior |
 |-----------|-----------|-------------|-------------------|
-| Volume does not exists | 5 NOT_FOUND | Indicates that a volume corresponding to the specified `volume_id` does not exist. | Caller MUST verify that the `volume_id` is correct and that the volume is accessible and has not been deleted before retrying with exponential back off. |
-| Node does not exists | 5 NOT_FOUND | Indicates that a node corresponding to the specified `node_id` does not exist. | Caller MUST verify that the `node_id` is correct and that the node is available and has not been terminated or deleted before retrying with exponential backoff. |
+| Volume does not exist | 5 NOT_FOUND | Indicates that a volume corresponding to the specified `volume_id` does not exist. | Caller MUST verify that the `volume_id` is correct and that the volume is accessible and has not been deleted before retrying with exponential back off. |
+| Node does not exist | 5 NOT_FOUND | Indicates that a node corresponding to the specified `node_id` does not exist. | Caller MUST verify that the `node_id` is correct and that the node is available and has not been terminated or deleted before retrying with exponential backoff. |
 | Volume published but is incompatible | 6 ALREADY_EXISTS | Indicates that a volume corresponding to the specified `volume_id` has already been published at the node corresponding to the specified `volume_id` but is incompatible with the specified `volume_capability` or `readonly` flag . | Caller MUST fix the arguments before retying. |
 | Volume published to another node | 9 FAILED_PRECONDITION | Indicates that a volume corresponding to the specified `volume_id` has already been published at another node and does not have MULTI_NODE volume capability. If this error code is returned, the Plugin SHOULD specify the `node_id` of the node at which the volume is published as part of the gRPC `status.message`. | Caller SHOULD ensure the specified volume is not published at any other node before retrying with exponential back off. |
 | Max volumes attached | 8 RESOURCE_EXHAUSTED | Indicates that the maximum supported number of volumes that can be attached to the specified node are already attached. Therefore, this operation will fail until at least one of the existing attached volumes is detached from the node. | Caller MUST ensure that the number of volumes already attached to the node is less then the maximum supported number of volumes before retrying with exponential backoff. |
@@ -952,8 +952,8 @@ The CO MUST implement the specified error recovery behavior when it encounters t
 
 | Condition | gRPC Code | Description | Recovery Behavior |
 |-----------|-----------|-------------|-------------------|
-| Volume does not exists | 5 NOT_FOUND | Indicates that a volume corresponding to the specified `volume_id` does not exist. | Caller MUST verify that the `volume_id` is correct and that the volume is accessible and has not been deleted before retrying with exponential back off. |
-| Node does not exists | 5 NOT_FOUND | Indicates that a node corresponding to the specified `node_id` does not exist. | Caller MUST verify that the `node_id` is correct and that the node is available and has not been terminated or deleted before retrying with exponential backoff. |
+| Volume does not exist | 5 NOT_FOUND | Indicates that a volume corresponding to the specified `volume_id` does not exist. | Caller MUST verify that the `volume_id` is correct and that the volume is accessible and has not been deleted before retrying with exponential back off. |
+| Node does not exist | 5 NOT_FOUND | Indicates that a node corresponding to the specified `node_id` does not exist. | Caller MUST verify that the `node_id` is correct and that the node is available and has not been terminated or deleted before retrying with exponential backoff. |
 | Operation pending for volume | 10 ABORTED | Indicates that there is a already an operation pending for the specified volume. In general the Cluster Orchestrator (CO) is responsible for ensuring that there is no more than one call "in-flight" per volume at a given time. However, in some circumstances, the CO MAY lose state (for example when the CO crashes and restarts), and MAY issue multiple calls simultaneously for the same volume. The Plugin, SHOULD handle this as gracefully as possible, and MAY return this error code to reject secondary calls. | Caller SHOULD ensure that there are no other calls pending for the specified volume, and then retry with exponential back off. |
 | Call not implemented | 12 UNIMPLEMENTED | ControllerUnpublishVolume call is not implemented by the plugin or disabled in the Plugin's current mode of operation. | Caller MUST NOT retry. Caller MAY call `ControllerGetCapabilities` or `NodeGetCapabilities` to discover Plugin capabilities. |
 
@@ -1000,7 +1000,7 @@ The CO MUST implement the specified error recovery behavior when it encounters t
 
 | Condition | gRPC Code | Description | Recovery Behavior |
 |-----------|-----------|-------------|-------------------|
-| Volume does not exists | 5 NOT_FOUND | Indicates that a volume corresponding to the specified `volume_id` does not exist. | Caller MUST verify that the `volume_id` is correct and that the volume is accessible and has not been deleted before retrying with exponential back off. |
+| Volume does not exist | 5 NOT_FOUND | Indicates that a volume corresponding to the specified `volume_id` does not exist. | Caller MUST verify that the `volume_id` is correct and that the volume is accessible and has not been deleted before retrying with exponential back off. |
 
 
 #### `ListVolumes`
@@ -1544,7 +1544,7 @@ The CO MUST implement the specified error recovery behavior when it encounters t
 
 | Condition | gRPC Code | Description | Recovery Behavior |
 |-----------|-----------|-------------|-------------------|
-| Volume does not exists | 5 NOT_FOUND | Indicates that a volume corresponding to the specified `volume_id` does not exist. | Caller MUST verify that the `volume_id` is correct and that the volume is accessible and has not been deleted before retrying with exponential back off. |
+| Volume does not exist | 5 NOT_FOUND | Indicates that a volume corresponding to the specified `volume_id` does not exist. | Caller MUST verify that the `volume_id` is correct and that the volume is accessible and has not been deleted before retrying with exponential back off. |
 | Operation pending for volume | 10 ABORTED | Indicates that there is a already an operation pending for the specified volume. In general the Cluster Orchestrator (CO) is responsible for ensuring that there is no more than one call "in-flight" per volume at a given time. However, in some circumstances, the CO MAY lose state (for example when the CO crashes and restarts), and MAY issue multiple calls simultaneously for the same volume. The Plugin, SHOULD handle this as gracefully as possible, and MAY return this error code to reject secondary calls. | Caller SHOULD ensure that there are no other calls pending for the specified volume, and then retry with exponential back off. |
 
 #### RPC Interactions and Reference Counting
@@ -1638,7 +1638,7 @@ The CO MUST implement the specified error recovery behavior when it encounters t
 
 | Condition | gRPC Code | Description | Recovery Behavior |
 |-----------|-----------|-------------|-------------------|
-| Volume does not exists | 5 NOT_FOUND | Indicates that a volume corresponding to the specified `volume_id` does not exist. | Caller MUST verify that the `volume_id` is correct and that the volume is accessible and has not been deleted before retrying with exponential back off. |
+| Volume does not exist | 5 NOT_FOUND | Indicates that a volume corresponding to the specified `volume_id` does not exist. | Caller MUST verify that the `volume_id` is correct and that the volume is accessible and has not been deleted before retrying with exponential back off. |
 | Volume published but is incompatible | 6 ALREADY_EXISTS | Indicates that a volume corresponding to the specified `volume_id` has already been published at the specified `target_path` but is incompatible with the specified `volume_capability` or `readonly` flag. | Caller MUST fix the arguments before retying. |
 | Operation pending for volume | 10 ABORTED | Indicates that there is a already an operation pending for the specified volume. In general the Cluster Orchestrator (CO) is responsible for ensuring that there is no more than one call "in-flight" per volume at a given time. However, in some circumstances, the CO MAY lose state (for example when the CO crashes and restarts), and MAY issue multiple calls simultaneously for the same volume. The Plugin, SHOULD handle this as gracefully as possible, and MAY return this error code to reject secondary calls. | Caller SHOULD ensure that there are no other calls pending for the specified volume, and then retry with exponential back off. |
 | Exceeds capabilities | 9 FAILED_PRECONDITION | Indicates that the CO has exceeded the volume's capabilities because the volume does not have MULTI_NODE capability. | Caller MAY choose to call `ValidateVolumeCapabilities` to validate the volume capabilities, or wait for the volume to be unpublished on the node. |
@@ -1683,7 +1683,7 @@ The CO MUST implement the specified error recovery behavior when it encounters t
 
 | Condition | gRPC Code | Description | Recovery Behavior |
 |-----------|-----------|-------------|-------------------|
-| Volume does not exists | 5 NOT_FOUND | Indicates that a volume corresponding to the specified `volume_id` does not exist. | Caller MUST verify that the `volume_id` is correct and that the volume is accessible and has not been deleted before retrying with exponential back off. |
+| Volume does not exist | 5 NOT_FOUND | Indicates that a volume corresponding to the specified `volume_id` does not exist. | Caller MUST verify that the `volume_id` is correct and that the volume is accessible and has not been deleted before retrying with exponential back off. |
 | Operation pending for volume | 10 ABORTED | Indicates that there is a already an operation pending for the specified volume. In general the Cluster Orchestrator (CO) is responsible for ensuring that there is no more than one call "in-flight" per volume at a given time. However, in some circumstances, the CO MAY lose state (for example when the CO crashes and restarts), and MAY issue multiple calls simultaneously for the same volume. The Plugin, SHOULD handle this as gracefully as possible, and MAY return this error code to reject secondary calls. | Caller SHOULD ensure that there are no other calls pending for the specified volume, and then retry with exponential back off. |