Update Aduino Harp dead link

Author: vmullet

README.md

@@ -35,7 +35,7 @@ Who uses RichTextFX?
 [mqtt-spy](http://kamilfb.github.io/mqtt-spy/)  
 [Alt.Text](http://alttexting.com/)  
 [Xanthic](https://github.com/jrguenther/Xanthic)  
-[Arduino Harp](http://www.avrharp.org/)  
+[Arduino Harp](https://www.youtube.com/watch?v=rv5raLcsPNs)  
 [Markdown Writer FX](https://github.com/JFormDesigner/markdown-writer-fx)  
 [OmniEditor](https://github.com/giancosta86/OmniEditor), which is then used by [Chronos IDE](https://github.com/giancosta86/Chronos-IDE)