message: Add `registry.default` example
diff: @@ -68,7 +68,12 @@ registry to use. For example, to publish the package in the current directory:
  Instead of always passing the `--registry` command-line option, the default
  registry may be set in [`.cargo/config.toml`][config] with the `registry.default`
  -key.
  +key. For example:
  +
  +```toml
  +[registry]
  +default = "my-registry"
  +```
  Setting the `package.publish` key in the `Cargo.toml` manifest restricts which
  registries the package is allowed to be published to. This is useful to
  