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 Document that in the book.
Merge pull request #416 from gaurish/master

Document TimeHelper::nice takes $format as param