Really fix the flutter dartdoc link (#22266)

It turns out that #21888 didn't really fix the link, because *this* is the README used by dartdoc for flutter.

dev/docs/README.md

@@ -8,4 +8,4 @@ source.
 * **Main site: [flutter.io](https://flutter.io/)**
 * [Install](https://flutter.io/setup/)
 * [Get started](https://flutter.io/getting-started/)
-* [Contribute](CONTRIBUTING.md)
+* [Contribute](https://github.com/flutter/flutter/blob/master/CONTRIBUTING.md#contributing-to-flutter)