minor symfony#9225 Added a missin namespace in CacheKernel example (javiereguiluz)

This PR was merged into the 4.0 branch.

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Added a missin namespace in CacheKernel example

This finishes symfony#9222 adding a missing `namespace App` in a `CacheKernel` example.

Commits
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e9b0a12 Added a missin namespace in CacheKernel example

Author: javiereguiluz

http_cache/cache_invalidation.rst

@@ -51,6 +51,7 @@ Here is how you can configure the Symfony reverse proxy (See :doc:`/http_cache`)
 to support the ``PURGE`` HTTP method::
 
     // src/CacheKernel.php
+    namespace App;
 
     use Symfony\Bundle\FrameworkBundle\HttpCache\HttpCache;
     use Symfony\Component\HttpFoundation\Request;