Update README.md demo image alt text (#3)

Author: kulesy

README.md

@@ -1,6 +1,6 @@
 # Tina Starter 🦙
 
-![tina-cloud-starter-demo](https://user-images.githubusercontent.com/103008/130587027-995ccc45-a852-4f90-b658-13e8e0517339.gif)
+![tina-nextjs-starter-demo](https://user-images.githubusercontent.com/103008/130587027-995ccc45-a852-4f90-b658-13e8e0517339.gif)
 
 This Next.js starter is powered by [TinaCMS](https://app.tina.io) for you and your team to visually live edit the structured content of your website. ✨