Readme Rust example without WASI?

I've tried things out for the first time, so I might be totally wrong. Hence I haven't opened a PR editing the readme, but I thought I ask first.

I've followed the steps from the [Guest: Rust section of the readme](https://github.com/bytecodealliance/wit-bindgen/tree/eeba5da107a992930c0f6b95a0d66b10ff69eebc#guest-rust). It starts with using `wasm32-wasi` as target. At the end you should use an adapter module to build the component. When I run `wasm-tools component wit my-component.wasm` I get lots of output that refers to WASI things. I guess that's correct.

But when I try to run it via Wasmtime, I get the error ```Error: import `wasi-stderr` not defined```.

What worked for me is: Using `wasm32-unknown-unknown` as target. I then don't add the adapter and simply run:

wasm-tools component new ./target/wasm32-wasi/debug/my-project.wasm -o my-component.wasm

The `wasm-tools component wit my-component.wasm` prints out what the readme says. When I run the resulting wasm file with Wasmime, it works as expected.