minor #3675 add missing code block directive (xabbuh)

This PR was merged into the 2.4 branch.

Discussion
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add missing code block directive

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.4+
| Fixed tickets |

Thought we already had a pull request for this but didn't find one.

Commits
-------

3ace4d6 add missing code block directive

cookbook/security/api_key_authentication.rst

@@ -214,11 +214,13 @@ Handling Authentication Failure
 -------------------------------
 
 In order for your ``ApiKeyAuthentication`` to correctly display a 403
-http status when either bad credentials or authentication fails you will 
-need to implement the :class:`Symfony\Component\Security\Http\Authentication\AuthenticationFailureHandlerInterface` on your 
-Authenticator. This will provide a method ``onAuthenticationFailure`` which 
+http status when either bad credentials or authentication fails you will
+need to implement the :class:`Symfony\Component\Security\Http\Authentication\AuthenticationFailureHandlerInterface` on your
+Authenticator. This will provide a method ``onAuthenticationFailure`` which
 you can use to create an error ``Response``.
 
+.. code-block:: php
+
     // src/Acme/HelloBundle/Security/ApiKeyAuthenticator.php
     namespace Acme\HelloBundle\Security;