Improve code examples in args parsing chapter

[stomar]

This adds more context to some examples in the "Parsing command line arguments" chapter.

[stomar]

E.g. instead of

let pattern = std::env::args().nth(1).expect("no pattern given");
let path = std::env::args().nth(2).expect("no path given");

now

fn main() {
    let pattern = std::env::args().nth(1).expect("no pattern given");
    let path = std::env::args().nth(2).expect("no path given");
}

This might help Rust beginners (like myself).