Add @types/mdx to app-dir-mdx example (#46523)

The `app-dir-mdx` is missing `@types/mdx` thus causing an error: `Cannot find module 'mdx/types' or its corresponding type...`

[Related PR](https://github.com/vercel/next.js/pull/45737) previously added types but was missing the line in `package.json`

## Bug

- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have a helpful link attached, see [`contributing.md`](https://github.com/vercel/next.js/blob/canary/contributing.md)

## Feature

- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [ ] [e2e](https://github.com/vercel/next.js/blob/canary/contributing/core/testing.md#writing-tests-for-nextjs) tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have a helpful link attached, see [`contributing.md`](https://github.com/vercel/next.js/blob/canary/contributing.md)

## Documentation / Examples

- [ ] Make sure the linting passes by running `pnpm build && pnpm lint`
- [ ] The "examples guidelines" are followed from [our contributing doc](https://github.com/vercel/next.js/blob/canary/contributing/examples/adding-examples.md)

examples/app-dir-mdx/package.json

@@ -8,6 +8,7 @@
   "dependencies": {
     "@next/font": "latest",
     "@next/mdx": "latest",
+    "@types/mdx": "2.0.3",
     "@types/node": "18.11.18",
     "@types/react": "18.0.27",
     "@types/react-dom": "18.0.10",