fix iso 8601 documentation in assignments api

refs CNVS-10087

test plan
 - rake doc:api should generate
 - assignments api documentation should have
   correct time stamp format

Change-Id: I6e891f7ab78e6f335de72d5f59bbd2eae9b0d5cc
Reviewed-on: https://gerrit.instructure.com/33785
QA-Review: August Thornton <august@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
Product-Review: Rob Orton <rob@instructure.com>