How to Hide the "Norse Runes Oracle Instructions/Setup" Admin Menu

function plt_hide_norse_runes_oracle_menus() {
	//Hide the "Tools → Norse Runes Oracle Instructions/Setup" menu.
	remove_submenu_page('tools.php', 'runes-submenu-page');
}

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

Where do I put this code?