Correct error in documentation

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.
Merge pull request #556 from jugglinmike/fix-map-docs

Correct error in documentation