Update README.md

Ensure the fiddle uses `let` instead of `var`.

README.md

@@ -64,7 +64,7 @@ function animate() {
 }
 ```
 
-If everything went well, you should see [this](https://jsfiddle.net/8kubjpL5/).
+If everything went well, you should see [this](https://jsfiddle.net/ruc3h17j/).
 
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