doc: Book uses Result::expect() but still makes reference to Option::expect()

[Sgeo]

https://doc.rust-lang.org/book/guessing-game.html contains statements like

If we leave off calling these two methods, our program will compile, but we’ll get a warning:

Luckily, if we just want to crash if there’s a problem, we can use these two little methods.

The book used to use .ok().expect(...), but has switched to .expect(...), from two methods for error handling/crashing to one.

[steveklabnik]

This has been fixed in nightly: https://doc.rust-lang.org/nightly/book/guessing-game.html

If we leave off calling this method, our program will compile, but we’ll get a warning:

[Sgeo]

The first line looks fixed, but the second doesn't, "we can use these two little methods" is still present.

[steveklabnik]

I thought I accepted PRs for all of it, will double check. Thanks.