Book chapter 1.2 futures::future::Future description

First, thanks a lot for this great library and it's accompanying documentation!

This description of `std::future::Future` form the book sounds not quite correct:

> In some sense, the std::future::Future can be seen as a minimal subset of futures::future::Future

https://book.async.rs/overview/std-and-library-futures.html

Actually both traits are the same. It's just a reexport:

https://github.com/rust-lang-nursery/futures-rs/blob/cde791c00b8b9c4fd14a594855038a1bc4b6323e/futures-core/src/future/mod.rs#L7