[BUGFIX] Fix wrong markup in warning message

Wrong closing tag in HTML has been fixed.

Change-Id: I5a701c86410eb0a4ea2ba55f43dcac182d7ac793
Releases: 3.0.0
Resolves: #58858
Reviewed-on: https://review.typo3.org/30167
Reviewed-by: Fabien Udriot
Tested-by: Fabien Udriot