Update MDX Guide config example (#34405)

The previous example may cause an unintentional change of the current `pageExtensions` setting, especially for the people who are not familiar with <https://nextjs.org/docs/api-reference/next.config.js/custom-page-extensions>

## Documentation / Examples

- [x] Make sure the linting passes by running `yarn lint`

docs/advanced-features/using-mdx.md

@@ -51,7 +51,8 @@ The following steps outline how to setup `@next/mdx` in your Next.js project:
      },
    })
    module.exports = withMDX({
-     pageExtensions: ['js', 'jsx', 'md', 'mdx'],
+     // Append the default value with md extensions
+     pageExtensions: ['ts', 'tsx', 'js', 'jsx', 'md', 'mdx'],
    })
    ```