fix the default api URL in judgehost

This API URL makes more sense because of the following two reasons:
1. As the the default commands given in the documentation install the domserver inside `$HOME/domjudge/domserver`.
2. The URL  should be appended with `index.php` otherwise `$_SERVER['PATH_INFO']` variable won't be set. See : http://stackoverflow.com/questions/5629683/serverpath-info-and-serverorig-path-info-in-php