Place parameters in enrollment object in create enrollment API example requests.

Fixes CNVS-14397

Test plan:
- rake doc:api
- verify that public/doc/api/enrollments.html#method.enrollments_api.create shows the correct api parameters

Change-Id: Ie22e65c6f47f05c3e3f7bd49d89757e47f2afeb2
Reviewed-on: https://gerrit.instructure.com/38501
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>