Indented two spaces to match the other things in this block

Change-Id: I87bd1cba830bd4c1de0136dd1f6b3636e74d12de

tox.ini

@@ -45,8 +45,8 @@ commands = {posargs}
 
 [testenv:docs]
 deps =
-   -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
-   -r{toxinidir}/doc/requirements.txt
+  -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+  -r{toxinidir}/doc/requirements.txt
 commands =
   rm -rf doc/build
   sphinx-build -W --keep-going -b html doc/source doc/build/html