Replace broken screencast thumbnail with embedded player.

README.md

@@ -48,7 +48,7 @@ borgmatic is hosted at <https://torsion.org/borgmatic> with [source code
 available](https://projects.torsion.org/witten/borgmatic). It's also mirrored
 on [GitHub](https://github.com/witten/borgmatic) for convenience.
 
-<a href="https://asciinema.org/a/203761" target="_blank"><img src="https://asciinema.org/a/203761.png" width="100%" /></a>
+<script src="https://asciinema.org/a/203761.js" id="asciicast-203761" async></script>
 
 
 ## Installation