minor #7612 Fix link for Twig Extensions documentation (mpdude)

This PR was submitted for the master branch but it was merged into the 2.7 branch instead (closes #7612).

Discussion
----------

Fix link for Twig Extensions documentation

See twigphp/Twig-extensions#201.

Commits
-------

cd40f4d Fix link for Twig Extensions documentation

Author: xabbuh

reference/twig_reference.rst

@@ -769,4 +769,4 @@ Those bundles can have other Twig extensions:
   :doc:`the Assetic Documentation </assetic/asset_management>`.
 
 .. _`Twig Reference`: http://twig.sensiolabs.org/documentation#reference
-.. _`the official Twig Extensions documentation`: http://twig.sensiolabs.org/doc/extensions/index.html
+.. _`the official Twig Extensions documentation`: http://twig-extensions.readthedocs.io/