[1.5.x] Fixed typo in docs/ref/signals.txt
[1.5.x] Fixed typo in docs/ref/signals.txt

Backport of 6fcb7ba842 from master

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