Update README.md

Signed-off-by: fulleni <fulleni@hotmail.com>

Author: fulleni

README.md

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-  <img src="https://avatars.githubusercontent.com/u/202675624?s=400&u=2daf23e8872a3b666bcd4f792a21fe2633097e79&v=4" alt="Flutter News App Toolkit Logo" width="220">
+  <img src="https://repository-images.githubusercontent.com/946589707/33b56f2c-76c3-4af0-a67f-8c08ca494b1b" alt="Flutter News App Toolkit Mockup" width="440">
   <h1>Flutter News App API Server</h1>
   <p><strong>Complete, production-ready source code for a Flutter news app api server.</strong></p>
 </div>