Outdated rustdoc for wasmtime-wasi

[wasmtime-wasi rustdoc](https://docs.rs/wasmtime-wasi/latest/wasmtime_wasi/index.html) currently says:

> wasmtime_wasi::Wasi::new(&Store, WasiCtx) is a struct which owns your WasiCtx and provides linkage to every available snapshot.

But it looks like `wasmtime_wasi::Wasi` struct doesn't exist anymore and, looking at [the other docs](https://docs.wasmtime.dev/examples-rust-wasi.html), seems like imports are provided differently nowadays.