An error with simple plugin?

Hi
I make a simple plugin:

<?php
/*
Plugin Name: xxxxxxx
*/
add_action('admin_menu', 'xxxxxx');
function xxxxxx() {
	add_submenu_page( 'themes.php', 'xxxxxx', 'xxxxxx', 'manage_options', 'xxxxxxx', 'xxxxxxx_callback' );
}

function xxxxxxx_callback() {
	echo 'xxxxxxxx';
}
?>

Then active with this Error. please help me:
The plugin generated 3 characters of unexpected output during activation. If you notice “headers already sent” messages, problems with syndication feeds or other issues, try deactivating or removing this plugin.