course[syllabus_body] added to api doc

fixes PS-853

Change-Id: I97e1e55f60cc803df90e94be5c1254a40447627f
Reviewed-on: https://gerrit.instructure.com/28494
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Bracken Mosbacker <bracken@instructure.com>
Product-Review: Bracken Mosbacker <bracken@instructure.com>
Product-Review: Adam Phillipps <adam@instructure.com>
QA-Review: Adam Phillipps <adam@instructure.com>