Require openstackdocstheme 1.18.1
Require openstackdocstheme 1.18.1

This update fixes bug URL links, require it.

Change-Id: I6212bb96c77ff69c10ba644d6b8d567154a15079

openstack/api-site,openstack/api-site,openstack/api-site,openstack/api-site