minor #15572 [Routing] Update routing.rst (authentictech)

This PR was submitted for the 5.3 branch but it was merged into the 4.4 branch instead.

Discussion
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[Routing] 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"

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Commits
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7c36043 Update routing.rst

Author: javiereguiluz

routing.rst

@@ -815,7 +815,8 @@ 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``) 
 
 Parameter Conversion
 ~~~~~~~~~~~~~~~~~~~~