Changed discussion section to read Assignment Comments

Changed the disscussion section inside the speedgrader to read
Assignment Comments instead of Discussion.

fixes CNVS-16756

test plan:
- As teacher create an assignment
- have student finish the assignment.
- Post a comment on the assignment under get details.
- have teacher click speedgrader.  Notice the
Assignment Comments section changed from Discussion

Change-Id: Iafd860cf8fba67aae8df1cabe4c2db4e6bbcfa64
Reviewed-on: https://gerrit.instructure.com/44297
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Benjamin Porter <bporter@instructure.com>
QA-Review: Sean Lewis <slewis@instructure.com>
Product-Review: Hilary Scharton <hilary@instructure.com>