message: Fix a typo in the unstable docs
diff: @@ -261,9 +261,9 @@ a `build.rs` script, you specify a list of build dependencies in the
  that runs each build dependency in order. Metabuild packages can then read
  metadata from `Cargo.toml` to specify their behavior.
  -Include `cargo-features` at the top of `Cargo.toml`, a `metadata` key in the
  +Include `cargo-features` at the top of `Cargo.toml`, a `metabuild` key in the
  `package`, list the dependencies in `build-dependencies`, and add any metadata
  -that the metabuild packages require. Example:
  +that the metabuild packages require under `package.metadata`. Example:
  ```toml
  cargo-features = ["metabuild"]
  