[1.8.x] Fixed typo in docs/ref/models/fields.txt
[1.8.x] Fixed typo in docs/ref/models/fields.txt

Backport of 53bc6e2e98b4889082049f50718b13f8d5d84b6b from master

h4r5h1t/django-hauthy,pquentin/django,h4r5h1t/django-hauthy,pquentin/django,pquentin/django,h4r5h1t/django-hauthy,h4r5h1t/django-hauthy