[BUGFIX] Fix text of t3skin update in Install-Tool

The description is currently returned as array but should
be a string. Furthermore the text needs to small update.

Change-Id: I93835ae6efefe308e685a1b3797444a6eabf27f7
Resolves: #53163
Releases: 6.2
Reviewed-on: https://review.typo3.org/25022
Reviewed-by: Stefan Neufeind
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters