fix md

Author: eryshkov

README.md

@@ -12,7 +12,7 @@
 <img src="https://img.shields.io/badge/licence-MIT-lightgray.svg" alt="Licence MIT"/>
 </p>
 
-[Demo video here]()
+[Demo video here](https://youtu.be/39ocQGBMGFU)
 
 ## Main functionality
 * simple sprite kit game app