[TASK] Rename "t3kin" CSS file

Fixes a funny typo. No real impact since all files in t3skin > visual are
automatically included anyways.

Kudos: Frank Spy-Eye Nägler

Resolves: #62924
Releases: master
Change-Id: I75e6b8556af6936eb4af6f0d52432bbf93b7c2c3
Reviewed-on: http://review.typo3.org/34092
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>