minor #20832 [Routing] Add missing type hint (silasjoisten)

This PR was submitted for the 7.2 branch but it was merged into the 6.4 branch instead.

Discussion
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[Routing] Add missing type hint

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Commits
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2153ebb Add type hint

Author: javiereguiluz

routing.rst

@@ -595,7 +595,7 @@ the ``{page}`` parameter using the ``requirements`` option:
             }
 
             #[Route('/blog/{slug}', name: 'blog_show')]
-            public function show($slug): Response
+            public function show(string $slug): Response
             {
                 // ...
             }