fixes #353: return message.payload (#354)

docs/source/getstarted.rst

@@ -166,7 +166,7 @@ Here we are using the `RunTask` strategy which runs a custom function over each
 
     def handle_message(message: Message[KafkaPayload]) -> Message[KafkaPayload]:
         print(f"MSG: {message.payload}")
-        return message
+        return message.payload
 
     class ConsumerStrategyFactory(ProcessingStrategyFactory[KafkaPayload]):
         """