fix 404 on upgrade guide

Author: goldensunliu

docs/guides/README.md

@@ -5,4 +5,4 @@
   * [Path Matching](/docs/guides/path-matching.md)
   * [Server Rendering](/docs/guides/server-rendering.md)
   * [Testing](/docs/guides/testing.md)
-  * [Upgrading to React Router 1.0](https://github.com/rackt/react-router/blob/master/UPGRADE_GUIDE.md)
+  * [Upgrading to React Router 1.0](https://github.com/rackt/react-router/blob/master/CHANGES.md)