Fix document formatting
Development of the WASI API has now moved to the WASI CG Subgroup; see
the [github repository], including the [API documentation].
-[github repository](https://github.com/WebAssembly/WASI/)
-[API documentation](https://github.com/WebAssembly/WASI/blob/master/phases/snapshot/docs.md)
+[github repository]: https://github.com/WebAssembly/WASI/
+[API documentation]: https://github.com/WebAssembly/WASI/blob/master/phases/snapshot/docs.md
