docs: fix code highlight in 07-fetching-data.mdx (#88727)
PR https://github.com/vercel/next.js/pull/82862 added an extra blank
line to the code snippet in
https://nextjs.org/docs/app/getting-started/fetching-data#parallel-data-fetching.
It should highlight the `Promise.all` line
Co-authored-by: Joseph <joseph.chamochumbi@vercel.com>
