fix Wasm example instructions

[FrankReh]

With these two new versions of the lines that state how to build the Wasm module and then how to run it with wasmtime, the Wasm demo of an mio TcpListener again works.

Note this crate still uses the wasi 0.11.0 dependency. The 0.12.0 and most recent 0.13.0 versions are built on a new WASI paradigm given the name WASI Preview 2, or WASI 0.2 for short. Those are likely going to require a lot more work again.

I'm just guessing the build will require the wasm32-wasip2 target but the std library doesn't build with that target yet, so this is just a small intermediate step.

Also worth noting is the recommended build step has been changed. Now +nightly is not needed and in fact causes the build to fail due to clippy warnings being treated as errors.