Add the missing doc/requirements into running-tests.rst

tox command install dependencies from the ``requirements.txt``,
``test-requirements.txt`` and ``doc/requirements.txt`` files

Change-Id: I21ee208d2484615ee3f9d9eca7602275382c920b

doc/source/contributor/running-tests.rst

@@ -18,8 +18,9 @@ Preferred way to run the tests
 
 The preferred way to run the unit tests is using ``tox``. It executes tests in
 isolated environment, by creating separate virtualenv and installing
-dependencies from the ``requirements.txt`` and ``test-requirements.txt`` files,
-so the only package you install is ``tox`` itself:
+dependencies from the ``requirements.txt``, ``test-requirements.txt`` and
+``doc/requirements.txt`` files, so the only package you install is ``tox``
+itself:
 
 .. code-block:: console