examples: update `next-config-js` document path (#73774)
## Description
Follow up #72465.
Update [`next-config-js`
document](https://nextjs.org/docs/canary/app/api-reference/config/next-config-js)
URL.
### Before
https://nextjs.org/docs/api-reference/next-config-js
### After
https://nextjs.org/docs/api-reference/config/next-config-js
### Adding or Updating Examples
- [x] The "examples guidelines" are followed from our contributing doc
https://github.com/vercel/next.js/blob/canary/contributing/examples/adding-examples.md
- [x] Make sure the linting passes by running `pnpm build && pnpm lint`.
See
https://github.com/vercel/next.js/blob/canary/contributing/repository/linting.md
CC: @samcx
Co-authored-by: Sam Ko <sam@vercel.com>
