Refactor: Discard unused next return value and update documentation in Chapter 13

[mavonarx]

This update refactors the build method in Listing 13-20 to explicitly discard the unused return value of the first use of args.next() in Chapter 13. To make it more clear the return value is not used.

[chriskrycho]

Thanks for the suggestion. What you suggested here would also be good and correct, but the text is actually just fine as is, and just calling args.next() without explicitly using let _ = with it is normal, idiomatic Rust, so I am going to leave it as is.