How to edit the notification email that admin receive when anyone register in the Wordpress MULTISITE

[admin]

I am working on a wordpress MU site.I am using domain mapping plugin.Now I want to modify the content of the notification email that admin receives when anyone register on the sites.I have searched in the internet and found function in pluggable.php in wp-includes folder of the wp installation.But event if i am overriding the function the mail content is still the same.
Here is the code:

if ( !function_exists('wp_new_user_notification') ) {
function wp_new_user_notification( $user_id, $plaintext_pass = '' ) {
    $user = new WP_User($user_id);

    $user_login = stripslashes($user->user_login);
    $user_email = stripslashes($user->user_email);

    $message  = sprintf(__('New user registration on your blog %s:'), get_option('blogname')) . "\r\n\r\n";
    $message .= sprintf(__('Username: %s'), $user_login) . "\r\n\r\n";
    $message .= sprintf(__('E-mail: %s'), $user_email) . "\r\n";
    $message .= "Testing message" . "\r\n";

    @wp_mail(get_option('admin_email'), sprintf(__('[%s] New User Registration'), get_option('blogname')), $message);

    if ( empty($plaintext_pass) )
        return;

    $message  = __('Hi there,') . "\r\n\r\n";
    $message .= sprintf(__("Welcome to %s! Here's how to log in:"), get_option('blogname')) . "\r\n\r\n";
    $message .= wp_login_url() . "\r\n";
    $message .= sprintf(__('Username: %s'), $user_login) . "\r\n";
    $message .= sprintf(__('Password: %s'), $plaintext_pass) . "\r\n\r\n";
    $message .= sprintf(__('If you have any problems, please contact me at %s.'), get_option('admin_email')) . "\r\n\r\n";
    $message .= __('Adios!');

    wp_mail($user_email, sprintf(__('[%s] Your username and password'), get_option('blogname')), $message);

}

}

Any help will be appriciated.Thanks!