select example in docs

[tungli]

I am an async noob but the 'simple example' in docs on select may be a bad example.
The example selects one of the futures:

let future1 = async { 1 };
let future2 = async { 2 };

The problem in this is that the computations are both Poll::Ready immediately (at least on my machine).
So the result of the select is always Either::Left, as I would expect from the implementation.
Therefore, the assert! in the example is misleading IMHO.

I would suggest being explicit about one future finishing first.
The simplest example I came up with is here.

[taiki-e]

I would accept a PR that improves this example.

[tungli]

Thanks @taiki-e! Created the PR #2345