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

Backport of 7614efa2f9e169749ab109aab909fbc244fd76a9 from master

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