minor #9722 Fix values class references (lex111)

This PR was submitted for the 4.0 branch but it was merged into the 2.7 branch instead (closes #9722).

Discussion
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Fix values class references

![image](https://user-images.githubusercontent.com/4408379/39536059-34c8191e-4e3e-11e8-89f1-788e11eddb4a.png)

Source - https://symfony.com/doc/current/reference/twig_reference.html#app

Commits
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a2ef02b Fix values class references

Author: javiereguiluz

reference/twig_reference.rst

@@ -762,8 +762,8 @@ needed objects and values. It is an instance of
 The available attributes are:
 
 * ``app.user``, a PHP object representing the current user;
-* ``app.request``, a :class:``Symfony\\Component\\HttpFoundation\\Request`` object;
-* ``app.session``, a :class:``Symfony\\Component\\HttpFoundation\\Session\\Session`` object;
+* ``app.request``, a :class:`Symfony\\Component\\HttpFoundation\\Request` object;
+* ``app.session``, a :class:`Symfony\\Component\\HttpFoundation\\Session\\Session` object;
 * ``app.environment``, a string with the name of the execution environment;
 * ``app.debug``, a boolean telling whether the debug mode is enabled in the app;
 * ``app.security`` (deprecated as of 2.6).