Fix broken argument for rewrites on link reference docs (#45073)

Fixes #45072

Rewrites don't accept relative paths anymore, we have to pass an absolute URL (see https://nextjs.org/docs/messages/middleware-relative-urls)



## Bug

- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have a helpful link attached, see [`contributing.md`](https://github.com/vercel/next.js/blob/canary/contributing.md)

## Feature

- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [ ] [e2e](https://github.com/vercel/next.js/blob/canary/contributing/core/testing.md#writing-tests-for-nextjs) tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have a helpful link attached, see [`contributing.md`](https://github.com/vercel/next.js/blob/canary/contributing.md)

## Documentation / Examples

- [x] Make sure the linting passes by running `pnpm build && pnpm lint`
- [x] The "examples guidelines" are followed from [our contributing doc](https://github.com/vercel/next.js/blob/canary/contributing/examples/adding-examples.md)

docs/api-reference/next/link.md

@@ -233,9 +233,9 @@ export function middleware(req) {
   const nextUrl = req.nextUrl
   if (nextUrl.pathname === '/dashboard') {
     if (req.cookies.authToken) {
-      return NextResponse.rewrite('/auth/dashboard')
+      return NextResponse.rewrite(new URL('/auth/dashboard', req.url))
     } else {
-      return NextResponse.rewrite('/public/dashboard')
+      return NextResponse.rewrite(new URL('/public/dashboard', req.url))
     }
   }
 }