Updated openstack/openstack
Project: openstack/keystone  5a1b6d93d5d049f2ae56085a27aec56287b67a35

remove unnecessary word in docs: 'an'

Per Brant's comment in:
    https://review.openstack.org/#/c/99075/3/doc/source/http-api.rst

Change-Id: I7d4b6d46666a71601f30da48bd720f847a8dfa09