Change API-WG guideline link to specs.o.o
Change API-WG guideline link to specs.o.o

This commit changes API-WG guideline link to spec.openstack.org instead
of github.com. We should use openstack.org links as possible.

Change-Id: Id0265bd6136a0c6f4adedf5c3aef34d34bbc933e

vedujoshi/tempest,openstack/tempest,cisco-openstack/tempest,cisco-openstack/tempest,openstack/tempest,vedujoshi/tempest,masayukig/tempest,Juniper/tempest,Juniper/tempest,masayukig/tempest