[1.5.x] Fixed #19374 -- Typo in docs/ref/models/instances.txt.
[1.5.x] Fixed #19374 -- Typo in docs/ref/models/instances.txt.

Backport of eeaab083f from master.

alx-eu/django,imtapps/django-imt-fork,bliti/django-nonrel-1.5,bliti/django-nonrel-1.5,imtapps/django-imt-fork,alx-eu/django,bliti/django-nonrel-1.5,hasadna/django,imtapps/django-imt-fork,alx-eu/django,hasadna/django,ccn-2m/django,ccn-2m/django,alx-eu/django,hasadna/django,ccn-2m/django,ccn-2m/django