async-await: fix examples

[taiki-e]

Motivation

Some examples in async-await fail to compile (even if updated to the new API by #1049).

Solution

• Fix crate path in Cargo.toml of examples
• Add edition2018 to examples in the documentation to make it compiled on Rust 2018
• Fix an example in the documentation

[taiki-e]

This is blocked on #1049.

[taiki-e]

To actually test the document, it needs to do one of the following in Cargo.toml of tokio-async-await:

1. Make async-await-preview feature dependent on "tokio/async-await-preview".
   This is not a problem when compiling on the root of the repository and tokio-async-await directory, but moving to directories of other subcrates and then compiling causes "failed to select a version for tokio-async-await" error.
2. Add features = ["async-await-preview"] to tokio dev-dependency.
   This will cause non-nightly tests to fail.

Both ways have problems, so they are not included in the first commit. (Probably, replacing that only in the build for async / await, it can testing on CI.)

[davidbarsky]

Thanks! merging this. I think @carllerche should be able to get a release out now that this and #1049 are in master.