[BUGFIX] Make path relative to alt_doc.php

Change-Id: I918da8508bd6ad27aa72559bf8574708dc413dc0
Releases: 3.2
Resolves: #61742
Reviewed-on: http://review.typo3.org/33305
Reviewed-by: Fabien Udriot <fabien.udriot@ecodev.ch>
Tested-by: Fabien Udriot <fabien.udriot@ecodev.ch>