bug symfony#5324 5259 improve 'Testing Documentation' in contributing guide (snoek09)

This PR was squashed before being merged into the 2.3 branch (closes symfony#5324).

Discussion
----------

5259 improve 'Testing Documentation' in contributing guide

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | symfony#5259

Commits
-------

c6ec1c8 5259 improve 'Testing Documentation' in contributing guide

Author: wouterj

contributing/documentation/format.rst

@@ -210,7 +210,7 @@ submitting your documentation, follow these steps:
 
 * Install Sphinx_;
 * Install the Sphinx extensions using git submodules: ``$ git submodule update --init``;
-* Run ``make html`` and view the generated HTML in the ``build/`` directory.
+* 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/