Fix wrong dir for doc/requirements.txt

The docu directory is called "doc", thus use "doc/requirements.txt".

Fix the typo in the name. Note that the PTI uses "doc" as well, see
https://governance.openstack.org/tc/reference/project-testing-interface.html#documentation

Change-Id: Ifd35cfdd37d43cf6e040727eb1f27472fe589afc

roles/ensure-sphinx/tasks/main.yaml

@@ -26,7 +26,7 @@
       source $VENV/bin/activate
       # skipping requirements.txt as it gets picked up by installing the
       # python package itself
-      for f in docs/requirements.txt test-requirements.txt ; do
+      for f in doc/requirements.txt test-requirements.txt ; do
           if [ -f $f ] ; then
               pip install $CONSTRAINTS -r $f
               break