api: fix docs for submission media_comment_id

test plan:
- generate the api docs
- look at the type of media_comment_id in the submission grade/comment endpoint
- it should be a string

Change-Id: I5d88edb04e7f836173099295877af795b22f00be
Reviewed-on: https://gerrit.instructure.com/35890
Reviewed-by: Eric Adams <eadams@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Duane Johnson <duane@instructure.com>
Product-Review: Duane Johnson <duane@instructure.com>
QA-Review: Jeremy Putnam <jeremyp@instructure.com>