minor #4791 Fix heading level in form_login_setup.rst (xelaris)

This PR was merged into the 2.3 branch.

Discussion
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Fix heading level in form_login_setup.rst

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.3+
| Fixed tickets |

Commits
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9d8a69f Fix heading level in form_login_setup.rst

cookbook/security/form_login_setup.rst

@@ -318,7 +318,7 @@ see :doc:`/cookbook/security/form_login`.
 .. _book-security-common-pitfalls:
 
 Avoid common Pitfalls
-=====================
+---------------------
 
 When setting up your login form, watch out for a few common pitfalls.
 
@@ -471,4 +471,4 @@ any firewall. This means you can't check for security or even access the
 user object on these pages. See :doc:`/cookbook/controller/error_pages`
 for more details.
 
-.. _`FOSUserBundle`: https://github.com/FriendsOfSymfony/FOSUserBundle
+.. _`FOSUserBundle`: https://github.com/FriendsOfSymfony/FOSUserBundle