Update syntax/indexmenu.php

workaround for the bug described here: 
https://www.dokuwiki.org/plugin:indexmenu?&#double_tree_is_shown_in_sidebar_of_upcoming_theme
Merge pull request #3 from geekspirit/patch-1

workaround for the bug described here: 
https://www.dokuwiki.org/plugin:indexmenu?&#double_tree_is_shown_in_sidebar_of_upcoming_theme