update with-mdx example to latest packages (#9186)

examples/with-mdx/next.config.js

-const withMDX = require('@zeit/next-mdx')({
+const withMDX = require('@next/mdx')({
   extension: /\.mdx?$/
 })
 module.exports = withMDX({

examples/with-mdx/package.json

@@ -7,8 +7,8 @@
     "start": "next start"
   },
   "dependencies": {
-    "@mdx-js/mdx": "^1.1.0",
-    "@zeit/next-mdx": "1.2.0",
+    "@mdx-js/loader": "^1.5.1",
+    "@next/mdx": "^9.1.1",
     "next": "latest",
     "react": "^16.8.6",
     "react-dom": "^16.8.6"