CsrfExtension namespace wrong in the documentation

[yamiko-ninja]

The documentation for the form component uses Symfony\Component\Security\Extension\Csrf\CsrfExtension which is not defined.

I believe the correct class is Symfony\Component\Form\Extension\Csrf\CsrfExtension.

Am I right? If so I can do a pull request to update the doc.

[xabbuh]

Good catch and welcome your pull request. :)