spec: fix sub-second precision error on assignments override spec

refs CNVS-7032

Change-Id: Ic480d1630550fb687c6fe59b12ad4d5621da8e89
Reviewed-on: https://gerrit.instructure.com/28863
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Brian Palmer <brianp@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>