doc: update how to run specific test in pytest
doc: update how to run specific test in pytest

pytest has a different way to run specific test from django test runner.
pytest migration patch did not update our document.
This commit is a follow-up patch.

Change-Id: Ifed388928f159a5a602088b2cfbf7f601d044387

openstack/horizon,ChameleonCloud/horizon,ChameleonCloud/horizon,ChameleonCloud/horizon,ChameleonCloud/horizon,openstack/horizon,openstack/horizon,openstack/horizon