fix typo in canvadoc url (and add specs)

fixes CNVS-15300

test plan:
- submit a pdf to an assignment (so it goes to crocodoc)
- go to the submission details page
- click 'View Feedback'
- it should preview in crocodoc

Change-Id: I7af38bbfa224e8a488b63d68d53fc234b90d6d48
Reviewed-on: https://gerrit.instructure.com/40684
Reviewed-by: Derek DeVries <ddevries@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>