Update CONTRIBUTING.md (#1732)

Fix #1731

Author: mit-mit

CONTRIBUTING.md

@@ -9,7 +9,7 @@ Yes!
 
 Start by [using the tool](README.md) and filing issues and requests.
 
-See the [dartdoc API](https://dartdoc.firebaseapp.com) docs, generated by
+See the [dartdoc API](https://pub.dartlang.org/documentation/dartdoc/latest/) docs, generated by
 dartdoc.
 
 If you want to contribute, check out the [issue tracker][] and see if there's an