update README with better looking youtube thumbnail

Author: Snailedlt

README.md

@@ -87,7 +87,7 @@ Options:
 
 ## Demo
 
-[![Demo Video](https://img.youtube.com/vi/7OJ4w5BCpQ0/0.jpg)](https://www.youtube.com/watch?v=7OJ4w5BCpQ0)
+[![Demo Video](http://markdown-videos-api.jorgenkh.no/youtube/7OJ4w5BCpQ0)](https://www.youtube.com/watch?v=7OJ4w5BCpQ0)
 
 ## Contributing