update demo link and screenshot

Author: ianstormtaylor

Readme.md

@@ -177,9 +177,9 @@ Which brings me to how Superstruct solves these issues...
 
 ### Demo
 
-Try out the [live demo on JSFiddle](https://jsfiddle.net/85nse1mk/) to get an idea for how the API works, or to quickly verify your use case:
+Try out the [live demo on CodeSandbox](https://codesandbox.io/s/bold-water-s2cr8d?file=/index.js) to get an idea for how the API works, or to quickly verify your use case:
 
-[![Demo screenshot.](./docs/images/demo-screenshot.png)](https://jsfiddle.net/85nse1mk/)
+[![Demo screenshot.](./docs/images/demo-screenshot.png)](https://codesandbox.io/s/bold-water-s2cr8d?file=/index.js)
 
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