Fix simple typo: properpty -> property (#717)
Fix simple typo: properpty -> property (#717)

Closes #716
cyberdelia/django-pipeline,jazzband/django-pipeline,beedesk/django-pipeline,d9pouces/django-pipeline,jazzband/django-pipeline,d9pouces/django-pipeline,cyberdelia/django-pipeline,beedesk/django-pipeline,beedesk/django-pipeline,jazzband/django-pipeline,cyberdelia/django-pipeline,d9pouces/django-pipeline