How to Hide the "MD Exporter" Admin Menu

function plt_hide_qh_markdown_exporter_menus() {
	//Hide the "Tools → MD Exporter" menu.
	remove_submenu_page('tools.php', 'qh-markdown-exporter');
}

add_action('admin_menu', 'plt_hide_qh_markdown_exporter_menus', 11);

Where do I put this code?