Update routing.rst

Just adding the ? character to the route parameter without also adding ? to the function parameter type results in a TypeError: "Argument 1 passed to App\Controller\BlogController::list() must be of the type integer, null given"

Author: authentictech

routing.rst

@@ -994,7 +994,8 @@ parameter:
 .. tip::
 
     To give a ``null`` default value to any parameter, add nothing after the
-    ``?`` character (e.g. ``/blog/{page?}``).
+    ``?`` character (e.g. ``/blog/{page?}``). Also, remember to add the
+    ``?`` character before the function parameter type to make it permit null values (e.g. ``?int``) 
 
 Priority Parameter
 ~~~~~~~~~~~~~~~~~~