docs: fix source files hash calculation (#54509)
Hello!
I recently tried to cache the `.next/cache` directory in a GitHub action following what was said in the documentation and realized that hashing the source files didn't work properly.
This problem also occured in [next-cache](https://github.com/jongwooo/next-cache) and was fixed by [this PR](https://github.com/jongwooo/next-cache/pull/17).
This PR simply changes the example from the documentation to apply the same fix (stop using brackets in the patterns passed to `hashFiles`).
Hope it helps!
### Improving Documentation
- [x] Run `pnpm prettier-fix` to fix formatting issues before opening the PR.
- [x] Read the Docs Contribution Guide to ensure your contribution follows the docs guidelines: https://nextjs.org/docs/community/contribution-guide
