message: Tweak wording of comment
diff: //! Another issue pointed out in #8364 was that `Cargo.lock` files were no
  //! longer compatible on stable and nightly with each other. The underlying
  //! issue is that Cargo was serializing `branch = "master"` *differently* on
  -//! nightly than it was on stable. Historical implementations of Cargo
  -//! desugared `branch = "master"` to having not dependency directives in
  -//! `Cargo.lock`, which means when reading `Cargo.lock` we can't differentiate
  -//! what's for the default branch and what's for the `master` branch.
  +//! nightly than it was on stable. Historical implementations of Cargo would
  +//! encode `DefaultBranch` and `Branch("master")` the same way in `Cargo.lock`,
  +//! so when reading a lock file we have no way of differentiating between the
  +//! two.
  //!
  //! To handle this difference in encoding of `Cargo.lock` we'll be employing
  //! the standard scheme to change `Cargo.lock`:
  