Changed "operation" to be required in Function Definitions (#631)

Signed-off-by: Helber Belmiro <[email protected]>

Author: hbelmiro

specification.md

@@ -3192,7 +3192,7 @@ Note that `transition` and `end` properties are mutually exclusive, meaning that
 | Parameter | Description | Type | Required |
 | --- | --- | --- | --- |
 | name | Unique function name | string | yes |
-| operation | If type is `rest`, <path_to_openapi_definition>#<operation_id>. If type is `asyncapi`, <path_to_asyncapi_definition>#<operation_id>. If type is `rpc`, <path_to_grpc_proto_file>#<service_name>#<service_method>. If type is `graphql`, <url_to_graphql_endpoint>#<literal \"mutation\" or \"query\">#<query_or_mutation_name>. If type is `odata`, <URI_to_odata_service>#<Entity_Set_Name>. If type is `expression`, defines the workflow expression. | string | no |
+| operation | If type is `rest`, <path_to_openapi_definition>#<operation_id>. If type is `asyncapi`, <path_to_asyncapi_definition>#<operation_id>. If type is `rpc`, <path_to_grpc_proto_file>#<service_name>#<service_method>. If type is `graphql`, <url_to_graphql_endpoint>#<literal \"mutation\" or \"query\">#<query_or_mutation_name>. If type is `odata`, <URI_to_odata_service>#<Entity_Set_Name>. If type is `expression`, defines the workflow expression. | string | yes |
 | type | Defines the function type. Can be either `rest`, `asyncapi`, `rpc`, `graphql`, `odata`, `expression`, or [`custom`](#defining-custom-function-types). Default is `rest` | enum | no |
 | authRef | References an [auth definition](#Auth-Definition) name to be used to access to resource defined in the operation parameter | string | no |
 | [metadata](#Workflow-Metadata) | Metadata information. Can be used to define custom function information | object | no |