add missing target

Author: xabbuh

security/form_login_setup.rst

@@ -377,3 +377,5 @@ As routing is done *before* security, 404 error pages are not covered by
 any firewall. This means you can't check for security or even access the
 user object on these pages. See :doc:`/controller/error_pages`
 for more details.
+
+.. _`FOSUserBundle`: https://github.com/FriendsOfSymfony/FOSUserBundle