minor #5185 Fix broken link in security chapter (iboved)

This PR was submitted for the 2.7 branch but it was merged into the 2.3 branch instead (closes #5185).

Discussion
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Fix broken link in security chapter

Hi everyone!

Q | A
------------ | -------------
Doc fix? | yes
New docs? | no
Applies to | 2.3, 2.5, 2.6, 2.7, 3.0
Fixed tickets | n/a

I was found broken link in security chapter and fixed it in this Pull Request.

Commits
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12df567 Fix broken link in security chapter

Author: wouterj

book/security.rst

@@ -1000,7 +1000,7 @@ the User object, and use the ``isGranted`` method (or
 Retrieving the User in a Template
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-In a Twig Template this object can be accessed via the `app.user <reference-twig-global-app>`_
+In a Twig Template this object can be accessed via the :ref:`app.user <reference-twig-global-app>`
 key:
 
 .. configuration-block::