fix optional/required API doc args

Logic was incorrect in the last update (SIS-399).

Test Plan:
- args marked as "optional" should be "required:false" in the
  API json
- args marked as "required" should be "required:true" in the
  API json

Fixes SIS-474

Change-Id: I5bb73c5a67fd820f1a73014437308825a3f1e2e3
Reviewed-on: https://gerrit.instructure.com/39735
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>