fix typo in extend_time controller

Change-Id: I2d0ab394b80a4b098339b45a8ae3afe59d575e68
Reviewed-on: https://gerrit.instructure.com/37911
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>