fix wrong grade calculator comment

Change-Id: I45709bd6dc2fd91bc8d178a38eaab6e088100445
Reviewed-on: https://gerrit.instructure.com/29011
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Simon Williams <simon@instructure.com>