message: Fix unstable sentence.
diff: @@ -27,10 +27,9 @@ name defined in a `Cargo.toml` manifest that can be checked within source
  code.
  Cargo has [*unstable feature flags*][cargo-unstable] which can be used to
  -enable experimental behavior of Cargo itself. The Rust compiler also has its
  -own unstable feature flags (see [The Unstable Book][unstable-book]) and
  -Rustdoc also has its own unstable feature flags (see [The Rustdoc
  -Book][rustdoc-unstable]).
  +enable experimental behavior of Cargo itself. The Rust compiler and Rustdoc
  +also have their own unstable feature flags (see [The Unstable
  +Book][unstable-book] and [The Rustdoc Book][rustdoc-unstable]).
  ### Index
  