fix: use union for operation action (#905)

spec/asyncapi.md

@@ -843,7 +843,7 @@ Describes a specific operation.
 
 Field Name | Type | Description
 ---|:---:|---
-<a name="operationObjectAction"></a>action | `string` | **Required**. Allowed values are `send` and `receive`. Use `send` when it's expected that the application will send a message to the given [`channel`](#operationObjectChannel), and `receive` when the application should expect receiving messages from the given [`channel`](#operationObjectChannel).
+<a name="operationObjectAction"></a>action | `"send"` &#124; `"receive"` | **Required**. Use `send` when it's expected that the application will send a message to the given [`channel`](#operationObjectChannel), and `receive` when the application should expect receiving messages from the given [`channel`](#operationObjectChannel).
 <a name="operationObjectChannel"></a>channel | [Reference Object](#referenceObject) | **Required**. A `$ref` pointer to the definition of the channel in which this operation is performed. Please note the `channel` property value MUST be a [Reference Object](#referenceObject) and, therefore, MUST NOT contain a [Channel Object](#channelObject). However, it is RECOMMENDED that parsers (or other software) dereference this property for a better development experience.
 <a name="operationObjectTitle"></a>title | `string` | A human-friendly title for the operation.
 <a name="operationObjectSummary"></a>summary | `string` | A short summary of what the operation is about.