removed image with 404 (#26376)

Update README.md to remove 404 link on Posse image

README.md

@@ -42,7 +42,7 @@ Cupertino (iOS-flavor) widgets, rich motion APIs,
 smooth natural scrolling, and platform awareness.
 
 [<img src="https://github.com/flutter/website/blob/master/src/images/homepage/screenshot-1.png" width="270" height="480" alt="Brand-first shopping design" align="left">](https://github.com/flutter/flutter/tree/master/examples/flutter_gallery/lib/demo/animation)
-[<img src="https://github.com/flutter/website/blob/master/src/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/src/images/homepage/screenshot-2.png" width="270" height="480" alt="Fitness app design">
 
 [<img src="https://github.com/flutter/website/blob/master/src/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/src/images/homepage/ios-friendlychat.png" width="270" height="480" alt="iOS chat app design">](https://codelabs.developers.google.com/codelabs/flutter/)