fix documentation error in context module items API

the content_id of an existing tag cannot be updated via PUT.

refs CNVS-14423

Change-Id: If97fbcc14b503a9771259e4d5e65a8e4c3b21113
Reviewed-on: https://gerrit.instructure.com/38540
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: James Williams  <jamesw@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>