Fix minor grammar error in get-started-identity
Fix minor grammar error in get-started-identity

This was recently refactored and a follow-on comment to merging the
patch asked for a minor grammar fix. The original review is here:

  https://review.openstack.org/#/c/370422/9/doc/common/get-started-identity.rst

Change-Id: Icc57fcd6098238114beb083276800faa1689d1c4

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