When reading this section as HTML, these double hyphens stood out to me. It looks like this was introduced last year in rust-lang#3135. I found other parts of the book where em dashes used standard Unicode em dash characters (rather than some Markdown representation) so I updated these to use them.

(Note that I actually think this would read more clearly if it used parentheses rather than em dashes, but I figured I'd start with the smallest possible suggestion.)