fix: use exit instead of return in the global scope

lib/exe/manifest.php

@@ -7,7 +7,7 @@ require_once(DOKU_INC . 'inc/init.php');
 
 if (!actionOK('manifest')) {
     http_status(404, 'Manifest has been disabled in DokuWiki configuration.');
-    return;
+    exit();
 }
 
 $manifest = new \dokuwiki\Manifest();