[1.6.x] Fixed #21218 -- Typo on docs/howto/upgrade-version.txt
[1.6.x] Fixed #21218 -- Typo on docs/howto/upgrade-version.txt

Thanks ryan at ryangallen.com for the report.

Backport of 36e220f923 from master

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