[BUGFIX] Incorrect class names used in Install.php script

Fixes an incorrect use of lowercase extension-key component
in "install" extension's Install.php script.

Releases: 6.2
Resolves: #53187
Change-Id: I67a9190fb861a9b22d6c83ea5147429ae76a729a
Reviewed-on: https://review.typo3.org/25042
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters