docs: align text definition of Message.payload with it's field definition (#983)

Author: char0n

spec/asyncapi.md

@@ -104,7 +104,7 @@ A consumer is a type of application, connected to a [server](#definitionsServer)
 
 ### <a name="definitionsMessage"></a>Message
 
-A message is the mechanism by which information is exchanged via a channel between [servers](#definitionsServer) and applications. A message MUST contain a payload and MAY also contain headers. The headers MAY be subdivided into [protocol](#definitionsProtocol)-defined headers and header properties defined by the application which can act as supporting metadata. The payload contains the data, defined by the application, which MUST be serialized into a format (JSON, XML, Avro, binary, etc.). Since a message is a generic mechanism, it can support multiple interaction patterns such as event, command, request, or response.
+A message is the mechanism by which information is exchanged via a channel between [servers](#definitionsServer) and applications. A message MAY contain a payload and MAY also contain headers. The headers MAY be subdivided into [protocol](#definitionsProtocol)-defined headers and header properties defined by the application which can act as supporting metadata. The payload contains the data, defined by the application, which MUST be serialized into a format (JSON, XML, Avro, binary, etc.). Since a message is a generic mechanism, it can support multiple interaction patterns such as event, command, request, or response.
 
 ### <a name="definitionsChannel"></a>Channel