Added fork me on github ribbon to demo page
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<body>
<script src="./bootstrap.js"></script>
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+ <a href="https://github.com/RustPython/RustPython"><img style="position: absolute; top: 0; right: 0; border: 0;" src="https://s3.amazonaws.com/github/ribbons/forkme_right_green_007200.png" alt="Fork me on GitHub"></a>
</body>
</html>
