Mention futures create right before before it is needed

[mdevino]

I was just following the Tokio tutorial and notice that, in the Async in depth section, the futures crate is mentioned before there are instructions to add it to Cargo.toml.

First occurrence:

But is only mentioned later in the tutorial:

This PR slightly changes the tutorial, so that the cargo add instructions are shown before the crate first usage.

Please let me know if further adjustments would be needed.