Fix incorrect xml tag in arch-design documentation.
Fix incorrect xml tag in arch-design documentation.

Change-Id: Ie0254c3a0c46927b9a9b123bdda5c3a2f618a9aa
Closes-Bug: #1434840
kairoaraujo/openstack-manuals,saeki-masaki/openstack-manuals,openstack/openstack-manuals,potsmaster/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,openstack/openstack-manuals,potsmaster/openstack-manuals,openstack/openstack-manuals,gongysh2004/openstack-manual,saeki-masaki/openstack-manuals,openstack/openstack-manuals,Akasurde/openstack-manuals,Akasurde/openstack-manuals,kairoaraujo/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,gongysh2004/openstack-manual