fix typo in sis api docs

Change-Id: Ibb303c49cb9c774bd1e8bacc7ad5ff297da4a96f
Reviewed-on: https://gerrit.instructure.com/31025
Reviewed-by: Rob Orton <rob@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Brian Palmer <brianp@instructure.com>
QA-Review: Brian Palmer <brianp@instructure.com>