Rename doc environment to docs

As per the governance resolution https://review.openstack.org/119875,
projects should have a docs environment in their tox.ini testing
interface. Rename the doc environment to docs.

Change-Id: I5908033d835d35a0650b77b01ff727d81eb79eb0
diff --git a/tox.ini b/tox.ini
index 3510ddb..6e45969 100644
--- a/tox.ini
+++ b/tox.ini
@@ -26,7 +26,7 @@
 commands =
   python setup.py testr --coverage
 
-[testenv:doc]
+[testenv:docs]
 commands = python setup.py build_sphinx
 
 [testenv:venv]