[Cache] Add return hint

Add Response return hint

Author: mohamedGasmii

http_cache.rst

@@ -215,7 +215,7 @@ The *easiest* way to cache a response is by caching it for a specific amount of
     use Symfony\Component\HttpFoundation\Response;
     // ...
 
-    public function index()
+    public function index(): Response
     {
         // somehow create a Response object, like by rendering a template
         $response = $this->render('blog/index.html.twig', []);