.sync() not found: Make more obvious that tokio is required.

[Corallus-Caninus]

as per this post
https://users.rust-lang.org/t/rusoto-how-to-make-sync-calls/41317
.sync() is not available for futures, specifically in my case for S3 object requests, but used throughout the documentation.

Tokio is required to handle the asynchronous futures but is not listed and should not be expected to be known since it is not in std and core.