Update caching doc as OccurenceOrderPlugin is applied by default in webpack2

Author: terryx

content/guides/caching.md

@@ -213,8 +213,7 @@ module.exports = {
     new ChunkManifestPlugin({
       filename: "chunk-manifest.json",
       manifestVariable: "webpackManifest"
-    }),
-    new webpack.optimize.OccurenceOrderPlugin()
+    })
   ]
 };
 ```