[BUGFIX] Collapse icon for accordion not centered in Install Tool

Resolves: #60192
Releases: 6.3,6.2
Change-Id: I4bacce041ca3be3dce4cae1a7407ab3fdf301ff0
Reviewed-on: https://review.typo3.org/31526
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters