Set pk_url_kwarg on tasks view so django knows where to look for task id.

Django generic views expect that if you are trying to retrieve an object by it's primary key (i.e. its id) it will be called with a url keyword named "pk". In this case the url keyword is "task_id", so we just need to tell it that. Once that was fixed I got a syntax error for the template due to a missing quotation mark - now fixed.