change outcomes index headings to be sequential from h1

fixes CNVS-13123

test plan:
- go to the course outcomes page
- enable a screenreader and view the headings list
- it should now have sequential headings, starting at h1 (specifically for this
  page, it should have an h1 and an h2)

Change-Id: I616d212c343df7b8e277b505aa00e93697ddda7b
Reviewed-on: https://gerrit.instructure.com/38255
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Anna Koalenz <akoalenz@instructure.com>
Reviewed-by: Cameron Sutter <csutter@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>