String used as a message in documentation

Changed message from 'How are you?' to {'msg': 'How are you?'}.

Another place in the doc says "The message itself must always be a
dict", so I'm assuming this was a mistake.

README.rst

@@ -193,7 +193,7 @@ propagate to the sender::
     actor_ref = Raiser.start()
 
     try:
-        actor_ref.ask('How are you?')
+        actor_ref.ask({'msg': 'How are you?'})
     except Exception as e:
         print(repr(e))
         # => Exception('Oops')