[TASK] Remove backslash for UnexpectedValueException in mod.php

Resolves: #58344
Releases: 6.2
Change-Id: Ic2cbc0a9d8a27c67e145ea9fef55c60d13466c33
Reviewed-on: https://review.typo3.org/29731
Reviewed-by: Oliver Klee
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn