[NVDA] Manage Announcement is no longer Manage discussion

Change screenreader text to say Manage Announcement

Fixes CNVS-13974

Test Plan:

- NVDA in FF
- Go to the Announcements page
- Select an Announcement
- Navigate to the Manage Announcements link (Gear icon that comes after the edit button)
- Verify it say manage announcement

Change-Id: I059cd0a6d339569729df90df25246282860ecf1e
Reviewed-on: https://gerrit.instructure.com/42398
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Mark Severson <markse@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Brad Horrocks <bhorrocks@instructure.com>