fix wording for grading levels

fixes #CNVS-12635

Per Erin Hallmark, "Grading Scheme" is more
conventional than "Grading Levels"

Test Plan:
 -Go to assignment details
 -Confirm the appropriate wording per the ticket

Change-Id: If2ada1072633c568145229da622c66c33a3b18ac
Reviewed-on: https://gerrit.instructure.com/39761
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>
QA-Review: Anna Koalenz <akoalenz@instructure.com>