fix api docs to display quiz submission user list

fixes CNVS-12914

test plan:
- to generate the documentation: run $rake doc:api
- visit /doc/api/index.html
- verify there is a link in the left nav for Quiz Submission User List

Change-Id: I908ccf9f7bc8c55a73d99b8af824bd10e3a4bcd1
Reviewed-on: https://gerrit.instructure.com/41741
Reviewed-by: Josh Simpson <jsimpson@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Spencer Olson <solson@instructure.com>