Correct error in documentation

This should resolve #555.

Commit message:

> The current documentation of the `map` method demonstrates an incorrect
> usage. Because the function creates a Cheerio object containing the
> values returned by the iterator function, the current example
> demonstrates creating a Cheerio object whose members are themselves
> Cheerio objects. The behavior of the `html` method invoked on such an
> object is not defined.
> 
> Update the example to demonstrate a simpler, more tradition use of the
> `map` method.