fix typo in gradebook2 settings menu

fixes CNVS-5298

test plan
- confirm treat ungraded as 0 setting works
- confirm treat ungraded as 0 is grammatically correct

Change-Id: I03ddd9a206578864a3da85efba44a27f09d0595c
Reviewed-on: https://gerrit.instructure.com/28935
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Liz Abinante <labinante@instructure.com>