[1.7.x] Fixed typo in docs/howto/custom-lookups.txt.
[1.7.x] Fixed typo in docs/howto/custom-lookups.txt.

Backport of f36ab2d3f2 from master

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