minor #4792 [BestPractices] add filename to codeblock (xelaris)

This PR was merged into the 2.3 branch.

Discussion
----------

[BestPractices] add filename to codeblock

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

Commits
-------

f210242 [BestPractices] add filename to codeblock

Author: wouterj

best_practices/security.rst

@@ -43,6 +43,7 @@ which uses a login form to load users from the database:
 
 .. code-block:: yaml
 
+    # app/config/security.yml
     security:
         encoders:
             AppBundle\Entity\User: bcrypt