Fix typo

Author: clemherreman

messenger.rst

@@ -266,7 +266,7 @@ matched under ``routing`` will still be handled immediately, i.e. synchronously.
 .. note::
 
     You may use ``'*'`` as the message class. This will act as a default routing
-    rule for any message not matched uner ``routing``. This is useful to ensure
+    rule for any message not matched under ``routing``. This is useful to ensure
     no message is handled synchronously by default.
 
 You can also route classes by their parent class or interface. Or send messages