fix markup in group category selector template

Test plan:
  group category selection on assignments/discussions should still work

Change-Id: I836a4842139358276ffb2ccb40591a616f4a8f6d
Reviewed-on: https://gerrit.instructure.com/32776
Reviewed-by: Simon Williams <simon@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Cameron Matheson <cameron@instructure.com>