fix 'assignment_visibility' API doc type

Swagger fails to generate code for pandarus / live API docs
because assignment_visibility array does not specify a type.

This sets the type to 'integer'.

Fixes SIS-400

Change-Id: I0a675dc51907d1a64993a36bed94d022deae3dda
Reviewed-on: https://gerrit.instructure.com/38575
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Ken Romney <kromney@instructure.com>
Product-Review: Duane Johnson <duane@instructure.com>
QA-Review: Jeremy Putnam <jeremyp@instructure.com>