[www] Fix link to COA
[www] Fix link to COA

The link changed on www.openstack.org, use new location.

Change-Id: Ic5e2e8e68ec2db8a828a46ac01bd6d803581c9e6

openstack/openstackdocstheme,openstack/openstackdocstheme,openstack/openstackdocstheme,openstack/openstackdocstheme