Try auto_core_update_email. E.g:

add_filter(\'auto_core_update_email\', function($email, $type, $core_update, $result)
{
  if (\'success\' === $type)
    $email[\'body\'] = sprintf( __(\'Congratulation! Your site at %1$s has been updated automatically to WordPress %2$s.\', \'textdomain\'), home_url(), $core_update->current ) . "\\n\\n";

  return $email;
}, PHP_INT_MAX, 4);