[1.6.x] Fixed typo in docs/ref/contrib/admin/index.txt.
[1.6.x] Fixed typo in docs/ref/contrib/admin/index.txt.

Backport of 06ba4b9eee from master

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