update button asset to use github repo

Author: Eric Liu

README.md

@@ -1,6 +1,6 @@
 # Node.js & SocketIO
 
-[![1-click-deploy](https://defang.io/deploy-with-defang.svg)](https://portal.defang.dev/redirect?url=https%3A%2F%2Fgithub.com%2Fnew%3Ftemplate_name%3Dsample-nodejs-chatroom-template%26template_owner%3DDefangSamples)
+[![1-click-deploy](https://raw.githubusercontent.com/DefangLabs/defang-assets/main/Logos/Buttons/SVG/deploy-with-defang.svg)](https://portal.defang.dev/redirect?url=https%3A%2F%2Fgithub.com%2Fnew%3Ftemplate_name%3Dsample-nodejs-chatroom-template%26template_owner%3DDefangSamples)
 
 This is a minimal chat application that shows how to use Socket.IO with Node.js with minimal code deployed with Defang.