Listing 20-20 Commentary improvement

[Yung-Beef]

URL to the section(s) of the book with this problem: https://doc.rust-lang.org/book/ch20-02-multithreaded.html

Description of the problem: This code block contains job() which seems to have appeared out of nowhere, and the text doesn't explain it. We never defined a job function, and it has a semicolon after it so the variable isn't being returned (and variables don't have parentheses after them anyways).

Suggested fix: Explain why job() is there, what type it is, what it's doing, why it has parentheses and is followed by a semicolon.

[chriskrycho]

This is explained extensively in the section immediately preceding it, beginning with Implementing the Execute Method. Listing 20-19 shows defining the job as a Box<dyn FnOnce() + Send + 'static> and creating a job of that type and sending it with self.sender.send(job).unwrap();, and then Listing 20-20 shows getting it in the receiver and using it.