[Toaster] [review-request] fix typo in models.py

[Barros Pena, Belen]

http://git.yoctoproject.org/cgit/cgit.cgi/poky-contrib/log/?h=bbarrosp/mode
ls-typo

Completely by chance, I found a typo that caused the tasks table to break
when a task has no help text.

Thanks

Belén