Fix CalendarEvent API docs

API docs must reference doc models in the same file, otherwise
they will be ignored by swagger codegen.

Test Plan:
- CalendarLink doc model should move from Calendar API doc page
  to Course API doc page.
- Calendar API doc page should show that CalendarEvent is returned
  by the get and list API calls.

Fixes SIS-247

Change-Id: Ic25e46462958388932b941dbaf8102258c1b1fb3
Reviewed-on: https://gerrit.instructure.com/33786
Reviewed-by: Eric Adams <eadams@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Duane Johnson <duane@instructure.com>
QA-Review: Jeremy Putnam <jeremyp@instructure.com>