fix oauth doc typo

Change-Id: Ic4ff44f2e62ee34e49c0f5e90e3eff83cc629703
Reviewed-on: https://gerrit.instructure.com/36475
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Rob Orton <rob@instructure.com>
Product-Review: Rob Orton <rob@instructure.com>
QA-Review: Rob Orton <rob@instructure.com>