How to Hide the "Markup Markdown" Admin Menu

function plt_hide_markup_markdown_menus() {
	//Hide the "Settings → Markup Markdown" menu.
	remove_submenu_page('options-general.php', 'markup-markdown-admin');
}

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

Where do I put this code?