fixes missing backtick

Author: Maxime Douailin

cookbook/security/pre_authenticated.rst

@@ -86,7 +86,7 @@ REMOTE_USER based Authentication
 .. versionadded:: 2.6
     REMOTE_USER pre authenticated firewall was introduced in Symfony 2.6.
 
-A lot of authentication modules, like ``auth_kerb` for Apache provide the username
+A lot of authentication modules, like ``auth_kerb`` for Apache provide the username
 using the ``REMOTE_USER`` environment variable. This variable can be trusted by
 the application since the authentication happened before the request reached it.