[1.6.x] Fixed typos in docs/ref/forms/validation.txt.
[1.6.x] Fixed typos in docs/ref/forms/validation.txt.

Backport of 66f5aa9fa5d53ddd7fbdb7ddac39c429f0c1b4fd from master

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