minor #17182 [4.4] fix code sample to log out (alexislefebvre)

This PR was merged into the 4.4 branch.

Discussion
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[4.4] fix code sample to log out

Backport of #17166

Commits
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46b8931 [4.4] fix code sample to log out

Author: xabbuh

security/form_login.rst

@@ -128,7 +128,7 @@ configuration (``login``):
             <route id="login" path="/login" controller="App\Controller\SecurityController::login" methods="GET|POST"/>
         </routes>
 
-    ..  code-block:: php
+    .. code-block:: php
 
         // config/routes.php
         use App\Controller\SecurityController;