fix header reading for group weights dialog

fixes CNVS-11101

test plan
=========
- navigate to the group weights dialog in screenreader gradebook or gb2
- verify that the column labels are read properly by the screenreader

Change-Id: Ia0f55e430f406818023f561071cc42ddf2a8cad5
Reviewed-on: https://gerrit.instructure.com/30207
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Reviewed-by: Aaron Cannon <acannon@instructure.com>
Product-Review: Matthew Irish <mirish@instructure.com>