Replaces npmcdn to upkg in README (remix-run#3785)

Author: Bilal Budhani

README.md

@@ -47,10 +47,10 @@ var Route = require('react-router').Route
 var Link = require('react-router').Link
 ```
 
-The UMD build is also available on [npmcdn](https://npmcdn.com):
+The UMD build is also available on [unpkg](https://unpkg.com):
 
 ```html
-<script src="https://npmcdn.com/react-router/umd/ReactRouter.min.js"></script>
+<script src="https://unpkg.com/react-router/umd/ReactRouter.min.js"></script>
 ```
 
 You can find the library on `window.ReactRouter`.