change comments to match the code

The example for the open/3 function called open! instead of open.

For the iterator/1 function, if you pass in a string instead of a device, the return value is
{ :ok, iterator_fn } instead of just iterator_fn, which breaks the example.  Since the comment
talks about converting a device to an iterator, it seemed appropriate to change to comment
to:

    { :ok, device } = File.open("README.md")
    source = File.iterator(device)

instead of changing it to:

    { :ok, source } = File.iterator("README.md")