[1.5.x] Fixed a typo in docs/topics/auth/customizing.txt
[1.5.x] Fixed a typo in docs/topics/auth/customizing.txt

Backport of 2c173ff3b4 from master

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