Fix non existing anchor in link

[diegorodriguezv]

The problem is that in https://rust-book.cs.brown.edu/ch05-01-defining-structs.html#borrowing-fields-of-a-structthe link to "Different Tuple Fields" takes you to the beginning of the chapter instead of the appropriate section.

I haven't used mdbook before so I'm not sure if this is the way to fix it. Let me know if there's a better way.

[willcrichton]

Also merged this in a bulk commit 780b7c9372a3581d08f4eeed814685b3dc753c2a.