Update README.md

README.md

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 Screenshot             |  Demo
 :-------------------------:|:-------------------------:
-<img src="tmux_screen.png" alt="drawing" width="400"/> |  <a href="http://www.youtube.com/watch?feature=player_embedded&v=Dd7ybndVGLQ" target="_blank"><img src="http://img.youtube.com/vi/Dd7ybndVGLQ/0.jpg" alt="IMAGE ALT TEXT HERE" width="400" height="400" border="10" /></a>
+<img src="tmux_screen.png" alt="drawing" width="400"/> |  [![Demo](https://img.youtube.com/vi/Dd7ybndVGLQ/0.jpg)](https://www.youtube.com/watch?v=Dd7ybndVGLQ "Demo tmux")