Fix QSQ API docs in Swagger

Closes CNVS-15813

test plan
---- ----

  - generate docs using `bundle exec rake doc:api`
  - go to localhost:3000/doc/api/live
  - open the console, reload the page, and verify you don't see an error
    like "Warning: quiz_questions is array type but has no 'items'"

Change-Id: Ic99fcc97c17fb26992ac93c13056c91f60936231
Reviewed-on: https://gerrit.instructure.com/41757
Reviewed-by: Derek DeVries <ddevries@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Duane Johnson <duane@instructure.com>
QA-Review: Duane Johnson <duane@instructure.com>