fix Assignment model in API Docs

An 'array' was specified without giving an 'items' key with type.
As a result, Pandarus can't build.

Fixes SIS-550

Change-Id: Id8cf296810ffc10da0ecab18af88af10f76bf05e
Reviewed-on: https://gerrit.instructure.com/40984
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Ethan Vizitei <evizitei@instructure.com>
Product-Review: Duane Johnson <duane@instructure.com>
QA-Review: Jeremy Putnam <jeremyp@instructure.com>