[BUGFIX] remove superfluous slash from path to grideditor.css

Change-Id: I7f625673e6f0593a39c0dfa0a38f2a6c3a3ef9f4
Resolves: #58788
Reviewed-on: https://review.typo3.org/30105
Reviewed-by: Jo Hasenau
Tested-by: Jo Hasenau