minor #16576 [Security] Update security.rst (mark2016)

This PR was submitted for the 6.0 branch but it was merged into the 4.4 branch instead.

Discussion
----------

[Security] Update security.rst

Added the config value for failure_path under form_login (it was missing).

<!--

If your pull request fixes a BUG, use the oldest maintained branch that contains
the bug (see https://symfony.com/releases for the list of maintained branches).

If your pull request documents a NEW FEATURE, use the same Symfony branch where
the feature was introduced (and `6.x` for features of unreleased versions).

-->

Commits
-------

fde7ca7 Update security.rst

Author: javiereguiluz

reference/configuration/security.rst

@@ -467,6 +467,14 @@ URL and process the submitted login credentials.
 Be sure that this URL is covered by your main firewall (i.e. don't create
 a separate firewall just for ``check_path`` URL).
 
+failure_path
+..........
+
+**type**: ``string`` **default**: ``/login``
+
+This is the route or path that the user is redirected to after a failed login attempt.
+It can be a relative/absolute URL or a Symfony route name.
+
 use_forward
 ...........