Document TimeHelper::nice takes $format as param 

As per the [TimeHelper::nice source code](https://github.com/cakephp/cakephp/blob/master/lib/Cake/Utility/CakeTime.php#L349), it can take $format as param. 

So Documented that in the [book](http://book.cakephp.org/2.0/en/core-libraries/helpers/time.html#TimeHelper::nice)