help-overlay: Mention Nintendo 3DS

The shortcuts are exactly same as for DS, so there's no point in
duplicating the page, but we should still mention it.
2 jobs for wip/exalm/3ds in 29 minutes and 20 seconds (queued for 1 second)
Status Name Job ID Coverage
  Test
passed flatpak #1033810
flatpak

00:29:20

passed tests #1033809
flatpak

00:04:48