Update code snippet for std::thread::spawn

[denispeplin]

The link: https://doc.rust-lang.org/stable/std/thread/fn.spawn.html

I'm not sure why the documentation is not linked from the book, maybe there's a reason for that.

[denispeplin]

It turns out, there is another copy of the same snippet on the same page: https://rust-book.cs.brown.edu/ch20-02-multithreaded.html#creating-space-to-store-the-threads

[willcrichton]

Thanks for the helpful change! If you can update the other snippet to match, I will merge this PR.