fixed wrong namespace length

as pointed out in
https://github.com/splitbrain/dokuwiki/pull/1530/files/b89dfc20f1e13333ce49ce0d6a3a83cc9836094b#r60392099

inc/load.php

@@ -117,7 +117,7 @@ function load_autoload($name){
     // plugin namespace
     if(substr($name, 0, 16) == 'dokuwiki/plugin/') {
         $name = str_replace('/test/', '/_test/', $name); // no underscore in test namespace
-        $file = DOKU_PLUGIN . substr($name, 7) . '.php';
+        $file = DOKU_PLUGIN . substr($name, 16) . '.php';
         if(file_exists($file)) {
             require $file;
             return true;