[1.6.x] Fixed grammatical error in auth docs
[1.6.x] Fixed grammatical error in auth docs

Backport of 51b50c47c7 from master

django-nonrel/django,redhat-openstack/django,felixjimenez/django,django-nonrel/django,redhat-openstack/django,felixjimenez/django,redhat-openstack/django,redhat-openstack/django,felixjimenez/django,dex4er/django,dex4er/django,felixjimenez/django,django-nonrel/django,dex4er/django,django-nonrel/django