Change CDN URL in README

[sadanora]

Hello.

Thank you for providing us with a great library.

The installation instructions for loading scripts from CDN had different versions in README and on the official website.

README: https://github.com/konvajs/konva#load-konva-via-classical-script-tag-from-cdn

Load Konva via classical <script> tag from CDN:.
<script src="https://unpkg.com/konva@8/konva.min.js"></script>

Official website: https://konvajs.org/docs/index.html

Or just use script tag:.
<script src="https://unpkg.com/konva@9/konva.min.js"></script>

I have modified the README to @9. Please merge if you are OK with it.

[lavrton]

Thanks.