minor #1617 fix rst markup (xabbuh)

This PR was merged into the 2.3.x-dev branch.

Discussion
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fix rst markup

Commits
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0519d457 fix rst markup

doc/providers/security.rst

@@ -426,7 +426,7 @@ switch back to their primary account:
         <a href="?_switch_user=_exit"> exit</a> the switch.
     {% endif %}
     
-Sharing security context between multiple firewalls
+Sharing Security Context between multiple Firewalls
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 By default, all the firewalls have a different **security context**. In case you
@@ -434,6 +434,8 @@ need to share the same security context between multiple firewalls you can set
 the ``context`` setting for each firewall you want the context to be shared
 with.
 
+.. code-block:: php
+
     $app['security.firewalls'] = array(
         'login' => array(
             'context' => 'admin_security',