Fix documentation for Kolla image-building guide
Fix documentation for Kolla image-building guide

A grammar error that might make it confusing for the readers
to follow the document was fixed.

Change-Id: I497822904d1360f64ef3fbaf9c128fcb38e2f8db

openstack/kolla,stackforge/kolla,rahulunair/kolla,rahulunair/kolla,stackforge/kolla,stackforge/kolla,openstack/kolla