Fix confusing wording about redeliver_timeout duration

Changed "slowest message duration" to "longest message duration" to clarify that
redeliver_timeout should be greater than the maximum processing time of any message.

Author: OskarStark

messenger.rst

@@ -1601,7 +1601,7 @@ The transport has a number of options:
 
     .. note::
 
-        Set ``redeliver_timeout`` to a greater value than your slowest message
+        Set ``redeliver_timeout`` to a greater value than your longest message
         duration. Otherwise, some messages will start a second time while the
         first one is still being handled.