minor #6397 Fix typo in SwitchUserListener file name (luxifer)

This PR was merged into the 2.3 branch.

Discussion
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Fix typo in SwitchUserListener file name

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.3 to 3.0
| Fixed tickets |

Commits
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c742190 Fix typo in SwitchUserListener file name

cookbook/security/impersonating_user.rst

@@ -223,7 +223,7 @@ how to change the sticky locale:
 
 .. code-block:: php
 
-        // src/AppBundle/EventListener/SwitchUserListener.pnp
+        // src/AppBundle/EventListener/SwitchUserListener.php
         namespace AppBundle\EventListener;
 
         use Symfony\Component\Security\Http\Event\SwitchUserEvent;