[BUGFIX] Typo in ValidationService::$reservedExtbaseNames

Reflectionexception -> ReflectionException

Change-Id: I4310f8dc5437df6d706255100a3f242a0f450b9d
Resolves: #58216
Releases: 6.2
Reviewed-on: https://review.typo3.org/29669
Reviewed-by: Nico de Haen
Tested-by: Nico de Haen