[Messenger] fix typo

Author: xabbuh

messenger.rst

@@ -316,7 +316,7 @@ to multiple transports:
 
 .. note::
 
-    If you configure routing for both a child and parent class, but rules
+    If you configure routing for both a child and parent class, both rules
     are used. E.g. if you have an ``SmsNotification`` object that extends
     from ``Notification``, both the routing for ``Notification`` and
     ``SmsNotification`` will be used.