message: Correct default cargo new edition
  I think this is a small typo in the docs but feel free to close this PR if I'm mistaken
diff: @@ -126,7 +126,7 @@ set to the latest edition. Setting the `edition` key in
  suites, benchmarks, binaries, examples, etc.
  If the `edition` key is not set to a specific [Rust Edition] in your
  -`Cargo.toml`, Cargo will default to 2015.
  +`Cargo.toml`, Cargo will default to 2018.
  ```toml
  [package]
  