update cover with beta

Author: sachaarbonel

README.md

@@ -9,7 +9,7 @@
    <a href="https://github.com/invertase/melos"><img alt="melos" src="https://img.shields.io/badge/maintained%20with-melos-f700ff.svg?style=flat-square"></a>
 </p>
 <p align="center">
-<img src="./images/flutter_feeds_alpha.png" alt="Stream Feed Activity Feed cover image" />
+<img src="./images/flutter_feeds_beta_1.png" alt="Stream Feed Activity Feed cover image" />
 </p>
 
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