Update README.md (#19717)

the thumbnail is not related to the link anymore 
https://github.com/flutter/flutter/issues/17380

README.md

@@ -31,7 +31,7 @@ smooth natural scrolling, and platform awareness.
 [<img src="https://github.com/flutter/website/blob/master/images/homepage/screenshot-2.png" width="270" height="480" alt="Fitness app design">](https://github.com/flutter/posse_gallery)
 
 [<img src="https://github.com/flutter/website/blob/master/images/homepage/screenshot-3.png" width="270" height="480" alt="Contact app design" align="left">](https://github.com/flutter/flutter/blob/master/examples/flutter_gallery/lib/demo/contacts_demo.dart)
-[<img src="https://github.com/flutter/website/blob/master/images/homepage/ios-friendlychat.png" width="270" height="480" alt="iOS chat app design">](https://codelabs.developers.google.com/codelabs/flutter-firebase)
+[<img src="https://github.com/flutter/website/blob/master/images/homepage/ios-friendlychat.png" width="270" height="480" alt="iOS chat app design">](https://codelabs.developers.google.com/codelabs/flutter/)
 
 Browse the <a href="https://flutter.io/widgets/">widget catalog</a>.