sets step value of default_grade input to allow decimals.

Fixes CNVS-4804

Test plan:
- In Chrome, select assignment in Gradebook2
- In the column header drop down, select "Set default grade"
- Enter a value such as 1.1
- No validation error should appear.

Change-Id: Ic2e43eb9135f782dc9e2c96f81aafed4efca4611
Reviewed-on: https://gerrit.instructure.com/33641
Reviewed-by: Cameron Matheson <cameron@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Coraline Ehmke <coraline@instructure.com>