[ticket/11821] Fix comma usage next to "You are receiving this notification".

This is a partial backport of 31502cdd6c3a5bac5164ec30530e5b4bf63f3196.

PHPBB3-11821
Merge remote-tracking branch 'bantu/ticket/11821' into develop-olympus

* bantu/ticket/11821:
  [ticket/11821] Fix comma usage next to "You are receiving this notification".