Create proper target path for shared storage fopen event

Signed-off-by: Morris Jobke <hey@morrisjobke.de>

apps/files_sharing/lib/SharedStorage.php

@@ -286,7 +286,7 @@ class SharedStorage extends \OC\Files\Storage\Wrapper\Jail implements ISharedSto
 				}
 		}
 		$info = [
-			'target' => $this->getMountPoint() . $path,
+			'target' => $this->getMountPoint() . '/' . $path,
 			'source' => $source,
 			'mode' => $mode,
 		];