[#2308] Reverting commit as 2.1 uses Twig 1.12, which uses the new way of doing extensions

Author: weaverryan

cookbook/templating/twig_extension.rst

@@ -118,6 +118,6 @@ Learning further
 For a more in-depth look into Twig Extensions, please take a look at the `Twig extensions documentation`_.
 
 .. _`Twig official extension repository`: https://github.com/fabpot/Twig-extensions
-.. _`Twig extensions documentation`: http://twig.sensiolabs.org/doc/advanced_legacy.html#creating-an-extension
+.. _`Twig extensions documentation`: http://twig.sensiolabs.org/doc/advanced.html#creating-an-extension
 .. _`global variables`: http://twig.sensiolabs.org/doc/advanced.html#id1
 .. _`functions`: http://twig.sensiolabs.org/doc/advanced.html#id2