[BUGFIX] fix typo in documentation

Change-Id: Ifbbf2106fe43c442bd9c64b610d6368e6902157f
Reviewed-on: http://review.typo3.org/32467
Reviewed-by: Carsten Biebricher <cb@hdnet.de>
Tested-by: Carsten Biebricher <cb@hdnet.de>