Fix typo in custom_db_role documentation (#780)

The Example Usage sections in the documentation correctly use actions.resources.#.collection_name as an attribute name, but the Actions section of the Arguments reference refers to resources.#.collection. The Example Usage section is correct.

Author: BryantD

website/docs/d/custom_db_role.html.markdown

@@ -59,7 +59,7 @@ Each object in the actions array represents an individual privilege action grant
 
 * `resources` - (Required) Contains information on where the action is granted. Each object in the array either indicates a database and collection on which the action is granted, or indicates that the action is granted on the cluster resource.
 
-* `resources.#.collection` - (Optional) Collection on which the action is granted. If this value is an empty string, the action is granted on all collections within the database specified in the actions.resources.db field.
+* `resources.#.collection_name` - (Optional) Collection on which the action is granted. If this value is an empty string, the action is granted on all collections within the database specified in the actions.resources.db field.
 
 * `resources.#.database_name`	Database on which the action is granted.
 

website/docs/d/custom_db_roles.html.markdown

@@ -55,7 +55,7 @@ Each object in the actions array represents an individual privilege action grant
 
 * `resources` - (Required) Contains information on where the action is granted. Each object in the array either indicates a database and collection on which the action is granted, or indicates that the action is granted on the cluster resource.
 
-* `resources.#.collection` - (Optional) Collection on which the action is granted. If this value is an empty string, the action is granted on all collections within the database specified in the actions.resources.db field.
+* `resources.#.collection_name` - (Optional) Collection on which the action is granted. If this value is an empty string, the action is granted on all collections within the database specified in the actions.resources.db field.
 
 * `resources.#.database_name`	Database on which the action is granted.
 

website/docs/r/custom_db_role.html.markdown

@@ -127,7 +127,7 @@ Each object in the actions array represents an individual privilege action grant
 
 * `resources` - (Required) Contains information on where the action is granted. Each object in the array either indicates a database and collection on which the action is granted, or indicates that the action is granted on the cluster resource.
 
-* `resources.#.collection` - (Optional) Collection on which the action is granted. If this value is an empty string, the action is granted on all collections within the database specified in the actions.resources.db field.
+* `resources.#.collection_name` - (Optional) Collection on which the action is granted. If this value is an empty string, the action is granted on all collections within the database specified in the actions.resources.db field.
 
 	-> **NOTE** This field is mutually exclusive with the `actions.resources.cluster` field.