remove unnecessary word in docs: 'an'
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

rodrigods/keystone,UTSA-ICS/keystone-kerberos,dstanek/keystone,jamielennox/keystone,reeshupatel/demo,maestro-hybrid-cloud/keystone,takeshineshiro/keystone,klmitch/keystone,openstack/keystone,blueboxgroup/keystone,mahak/keystone,openstack/keystone,roopali8/keystone,promptworks/keystone,vivekdhayaal/keystone,mahak/keystone,rodrigods/keystone,nuxeh/keystone,jumpstarter-io/keystone,rajalokan/keystone,ilay09/keystone,MaheshIBM/keystone,roopali8/keystone,promptworks/keystone,jumpstarter-io/keystone,mahak/keystone,promptworks/keystone,rajalokan/keystone,dims/keystone,ilay09/keystone,blueboxgroup/keystone,himanshu-setia/keystone,cernops/keystone,idjaw/keystone,reeshupatel/demo,jonnary/keystone,nuxeh/keystone,ilay09/keystone,openstack/keystone,dstanek/keystone,vivekdhayaal/keystone,himanshu-setia/keystone,jonnary/keystone,nuxeh/keystone,maestro-hybrid-cloud/keystone,ging/keystone,vivekdhayaal/keystone,MaheshIBM/keystone,dims/keystone,rushiagr/keystone,takeshineshiro/keystone,ging/keystone,cernops/keystone,rushiagr/keystone,jamielennox/keystone,jumpstarter-io/keystone,reeshupatel/demo,dstanek/keystone,UTSA-ICS/keystone-kerberos,klmitch/keystone,idjaw/keystone,ajayaa/keystone,ajayaa/keystone,rajalokan/keystone,rushiagr/keystone