respondus: link assignment to quiz correctly

fixes CNVS-8915

test plan:
- use the respondus quiz builder tool to create a quiz
- in the settings tab, check 'publish now'
- export the quiz to canvas
- take the quiz as a student
- the score you get should show up in the gradebook

Change-Id: I36dd1046b1ee1b82a7ad4bafac2a7e53dd309b59
Reviewed-on: https://gerrit.instructure.com/32617
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Stanley Stuart <stanley@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Reviewed-by: Bracken Mosbacker <bracken@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>