bug #6606 fix #6602 (yamiko-ninja)

This PR was submitted for the master branch but it was merged into the 2.7 branch instead (closes #6606).

Discussion
----------

fix #6602

Fixes namespace for Extension in the docs

Commits
-------

1a00342 fix #6602

components/form/introduction.rst

@@ -121,7 +121,7 @@ The following snippet adds CSRF protection to the form factory::
 
     use Symfony\Component\Form\Forms;
     use Symfony\Component\HttpFoundation\Session\Session;
-    use Symfony\Component\Security\Extension\Csrf\CsrfExtension;
+    use Symfony\Component\Form\Extension\Csrf\CsrfExtension;
     use Symfony\Component\Security\Csrf\TokenStorage\SessionTokenStorage;
     use Symfony\Component\Security\Csrf\TokenGenerator\UriSafeTokenGenerator;
     use Symfony\Component\Security\Csrf\CsrfTokenManager;