Wrong link to WASI Preview 2 in README.md file

The link to WASI Preview 2 in the README.md file goes to the non-existent

https://github.com/WebAssembly/WASI/tree/main/preview2

rather than the correct

https://github.com/WebAssembly/WASI/tree/main/wasip2

[sunfishcode]

Thanks! I've now filed #404 to fix this.