minor #15738 Fix method link to isCsrfTokenValid (mathieu-ducrot)

This PR was merged into the 4.4 branch.

Discussion
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Fix method link to isCsrfTokenValid

## Changelog

Fix the method link from AbstractController.php to ControllerTrait.php because the method comes from the Trait.

Commits
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d337720 Fix method link to isCsrfTokenValid

Author: javiereguiluz

security/csrf.rst

@@ -150,7 +150,7 @@ generate a CSRF token in the template and store it as a hidden form field:
     </form>
 
 Then, get the value of the CSRF token in the controller action and use the
-:method:`Symfony\\Bundle\\FrameworkBundle\\Controller\\AbstractController::isCsrfTokenValid`
+:method:`Symfony\\Bundle\\FrameworkBundle\\Controller\\ControllerTrait::isCsrfTokenValid`
 to check its validity::
 
     use Symfony\Component\HttpFoundation\Request;