message: refine rustc build instructions
diff: @@ -50,7 +50,7 @@ git clone https://github.com/rust-lang/rust/ rustc
  cd rustc
  cp config.toml.example config.toml
  # Now edit `config.toml` and set `debug-assertions = true`
  -./x.py build
  +./x.py build src/rustc
  rustup toolchain link custom build/x86_64-unknown-linux-gnu/stage2
  ```
  The `build` step can take 30 to 60 minutes.
  