Fix missing PagesOnly in 05-mdx (#64505)
Looks like we have a syntax error in
https://github.com/vercel/next.js/pull/63568 from missing closing
`</PagessOnly>` tag.
```sh
b383-75ab3d452c93.mdx:0:0: ERROR: [plugin: @mdx-js/esbuild] Expected a closing tag for `</PagesOnly>` (503:1-503:12)
```
x-ref: https://github.com/vercel/next.js/pull/63568
Closes NEXT-3111
