Several times usefull is used instead of useful.

Change-Id: Id8377d99fd94a20f4dd6261069f3b3dbb417d21d
Fixes: #39499
Releases: 6.0
Reviewed-on: http://review.typo3.org/13431
Reviewed-by: Stefan Busemann
Tested-by: Stefan Busemann
Reviewed-by: Mattias Nilsson
Tested-by: Mattias Nilsson
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer