switch documentation job to new PTI

This is a mechanically generated patch to switch the documentation
jobs to use the new PTI versions of the jobs as part of the
python3-first goal.

See the python3-first goal document for details:
https://governance.openstack.org/tc/goals/stein/python3-first.html

Depends-On: https://review.openstack.org/598472
Change-Id: I79e5e84735d4e1705ab5bf10d8e0fc2296933d30
Story: #2002586
Task: #24305

.zuul.d/ubuntu.yaml

@@ -4,8 +4,8 @@
       - openstack-python-jobs
       - openstack-python35-jobs
       - check-requirements
-      - publish-openstack-sphinx-docs
-      - release-notes-jobs
+      - publish-openstack-docs-pti
+      - release-notes-jobs-python3
       - periodic-stable-jobs
     check:
       jobs: