Fix incorrect xml file path in ZIP creation

phpcs-errors:0 phpunit-status:successful phpstan-errors:1

public/index6.php

@@ -39,7 +39,7 @@
 
     $y++;
 
-    $zieldatei = 'xml/' . $y . '.xml';
+    $zieldatei = __DIR__ . '/../xml/' . $y . '.xml';
     if (!$handle = fopen($zieldatei, 'w')) {
         echo "Cannot open file ($zieldatei)";exit;
     }