docs: update broken link in UPGRADING.md (#60342)
This link →
[/docs/02-app/01-building-your-application/09-upgrading/index.mdx](https://github.com/vercel/next.js/blob/21452921c055421ec8a44dae6e0e5f311c503337/docs/02-app/01-building-your-application/09-upgrading/index.mdx)
doesn't exist.
Fixing it to link to the correct directory.
Closes NEXT-2006
