[BUGFIX] Fix system maintainer checkbox size

Change-Id: Ibdd234c7f6c32c5edb9af25ea5e5e74ef91feb57
Resolves: #83690
Releases: master
Reviewed-on: https://review.typo3.org/55458


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: Frank Naegler <frank.naegler@typo3.org>
Reviewed-by: Susanne Moog <susanne.moog@typo3.org>
Tested-by: Susanne Moog <susanne.moog@typo3.org>