Update openstackdocstheme.

Update to the latest version.

Change-Id: Ic861e69a6bc268c30fead9ec1a1f6a9a735f7403

doc/requirements.txt

@@ -1,5 +1,5 @@
 # this is required for the docs build jobs
 sphinx>=2.0.0,!=2.1.0 # BSD
-openstackdocstheme>=2.2.1 # Apache-2.0
+openstackdocstheme>=2.2.2 # Apache-2.0
 doc8>=0.8.0 # Apache-2.0
 sphinxcontrib-apidoc>=0.2.0  # BSD