CONTRIBUTING: use "dart format" instead of "dartfmt"

CONTRIBUTING.md

@@ -11,7 +11,7 @@ If you'd like to add a feature or fix a bug, we're more than happy to accept pul
   - Ensure your code contains no analyzer errors, e.g.
     - Code is strong-mode compliant
     - Code is free of lint errors
-  - Format your code with `dartfmt`
+  - Format your code with `dart format`
   - Write tests for all new code paths, consider using the [Stream Matchers](https://pub.dartlang.org/packages/test#stream-matchers) available from the test package.
   - Write helpful documentation
   - If you would like to make a bigger / fundamental change to the codebase, please file a lightweight example PR / issue, or contact us in [Gitter](https://gitter.im/ReactiveX/rxdart) so we can discuss the issue.