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

Backport of 53bc6e2e98b4889082049f50718b13f8d5d84b6b from master

jyotsna1820/django,oscaro/django,oscaro/django,jyotsna1820/django,jyotsna1820/django,jyotsna1820/django,oscaro/django,oscaro/django