docs/notifications: fixup sending a notification

Dependencies

• [2] 6A5QEB4Q Write up some documentation on notifications
• [3] OTJ3WFLG notifications: Document an example scenario where builds_finished is triggered twice.

Change contents

• replacement in doc/manual/src/notifications.md at line 43
  [2.2277]→[2.2277:2398](∅→∅)
  − Notifications can be experimentally re-sent on the command line with `psql`, with `NOTIFY $notificationname '$payload'`.
  [2.2277]

  [2.2398]
  + Notifications can be experimentally re-sent on the command line with `psql`, with `NOTIFY $notificationname, '$payload'`.