Fix testing instructions

Author: michalpipa

contributing/documentation/format.rst

@@ -209,11 +209,12 @@ Nevertheless, if you prefer to do this check locally on your own machine before
 submitting your documentation, follow these steps:
 
 * Install Sphinx_;
-* Install the Sphinx extensions using git submodules: ``$ git submodule update --init``;
+* Install `Sphinx Extensions for PHP and Symfony`_;
 * Run ``make html`` and view the generated HTML in the ``_build/html`` directory.
 
 .. _reStructuredText: http://docutils.sourceforge.net/rst.html
 .. _Sphinx: http://sphinx-doc.org/
+.. _`Sphinx Extensions for PHP and Symfony`: https://github.com/fabpot/sphinx-php
 .. _`Symfony documentation`: https://github.com/symfony/symfony-docs
 .. _`reStructuredText Primer`: http://sphinx-doc.org/rest.html
 .. _`reStructuredText Reference`: http://docutils.sourceforge.net/docs/user/rst/quickref.html