fix admin controller index api find

fixes CNVS-16807

test plan
 - account admin index should find user by sis_id

Change-Id: Ife67825c388c9d08802e12e2bfc2155aee573da1
Reviewed-on: https://gerrit.instructure.com/44219
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Shawn Meredith <shawn@instructure.com>
Product-Review: Rob Orton <rob@instructure.com>
QA-Review: Rob Orton <rob@instructure.com>