message: doc: Use absolute url in prerequisites index
diff: To run `wasm-pack` you'll need to have both Rust and npm installed and configured.
  -- [Rust](./prerequisites/rust.html)
  -- [npm](./prerequisites/npm.html)
  +- [Rust](/wasm-pack/book/prerequisites/rust.html)
  +- [npm](/wasm-pack/book/prerequisites/npm.html)
  In the future, we intend to rewrite the npm registry client bits so that the need
  for a Node runtime is eliminated. If you're excited about that work- you should
  