Quiz Submission Questions - fix serializer lookup

Closes CNVS-14021

TEST PLAN
---- ----

  - create a quiz with a question
  - try answering the quiz using the QSQ API (see docs)
  - verify that your answer is recorded

Change-Id: I667671f639ac0b5c6bc469477b638e70f90f37ff
Reviewed-on: https://gerrit.instructure.com/37372
Reviewed-by: Derek DeVries <ddevries@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Derek DeVries <ddevries@instructure.com>