docs(javascript): fix wrong javascript code in `11-parallel-routes.mdx` (#71074)
## 📚 Improving Documentation
Hello, I've fixed an wrong JavaScript code in
[`11-parallel-routes.mdx`](https://nextjs.org/docs/app/building-your-application/routing/parallel-routes).
There's no need to use type declaration in JavaScript code. So I deleted
it.
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Co-authored-by: Zack Tanner <1939140+ztanner@users.noreply.github.com>