docs(spec): fix the type and decs of Operation.message field

Refs asyncapi#588

spec/asyncapi.md

@@ -668,7 +668,7 @@ Field Name | Type | Description
 <a name="operationObjectExternalDocs"></a>externalDocs | [External Documentation Object](#externalDocumentationObject) | Additional external documentation for this operation.
 <a name="operationObjectBindings"></a>bindings | [Operation Bindings Object](#operationBindingsObject) \| [Reference Object](#referenceObject) | A map where the keys describe the name of the protocol and the values describe protocol-specific definitions for the operation.
 <a name="operationObjectTraits"></a>traits | [[Operation Trait Object](#operationTraitObject) &#124; [Reference Object](#referenceObject) ] | A list of traits to apply to the operation object. Traits MUST be merged into the operation object using the [JSON Merge Patch](https://tools.ietf.org/html/rfc7386) algorithm in the same order they are defined here.
-<a name="operationObjectMessage"></a>message | [[Message Object](#messageObject) &#124; [Reference Object](#referenceObject)] | A definition of the message that will be published or received on this channel. `oneOf` is allowed here to specify multiple messages, however, **a message MUST be valid only against one of the referenced message objects.**
+<a name="operationObjectMessage"></a>message | [Message Object](#messageObject) &#124; [Reference Object](#referenceObject) &#124; Map["oneOf", [[Message Object](#messageObject) &#124; [Reference Object](#referenceObject)]] | A definition of the message that will be published or received on this channel. Map containing a single `oneOf` key is allowed here to specify multiple messages. However, **a message MUST be valid only against one of the message objects.**
 
 This object can be extended with [Specification Extensions](#specificationExtensions).