Change Array.iter to Array.map

In the chapter 13, you said I need to implement a double function that doubles all the values of an array with `Array.iter`. The function that `Array.iter` takes must need to be type unit. So the correct way of doing that is with `Array.map`.

Start with the chapter 13 exercise that asks for a double function using Array.iter. Update the instructional text to refer to Array.map, and verify that the example describes an array transformation rather than a unit-returning iteration.

Written by the indexing model from the issue text.