Found a couple of documentation bugs

[pgoldweic]

Hi, as I'm migrating from an older version of this middleware to 2.0.1, I've noticed what appear to be a couple of documentation bugs to me (in the readme file), as follows:

1- Everywhere where 'message_received' is used, should become 'message' (this is a Botkit change)
2- The middleware :before and :after signatures appear to be incorrect. I have only tested the 'after' method and not the 
'before', but I suspect that the same problem might apply to both. Specifically, instead of being:
 middleware.after = (message, assistantResponse) => async () => {
       // Code here gets executed after the call to Assistant.
       return assistantResponse;
  });
  we really want it to be: 
 middleware.after = async (message, assistantResponse) => {
       // Code here gets executed after the call to Assistant.
       return assistantResponse;
  });

(note the difference in signatures above for item #2). Following the signature used in the docs, my bot would be essentially mute, and 'watsonData' in a message ended up having as value the async function itself (odd), while the second signature (more in line with the one used in earlier versions) produces bot responses as expected.

[Naktibalda]

Please raise pull request