Fix typo and sources in WORKING_GROUPS.md
Therefor is a typo of therefore, and was fixed. There were also two
places where the website WG was directly linked, where they should have
put the WG's name/repo; that was fixed as well.
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
Therefor is a typo of therefore, and was fixed. There were also two
places where the website WG was directly linked, where they should have
put the WG's name/repo; that was fixed as well.