[www] fix errorpage links
[www] fix errorpage links

Fix URLs in errorpage, due to missing "/" they linked to e.g.
https://docs.openstack.org/www.openstack.org/software instead of
https://www.openstack.org/software.

Change-Id: Id97ec25b427a461c77bd2065f1d0c4d6eb423d89

openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals