Update Convex example to convex 0.1.9 (#40162)
## Documentation / Examples
Update Convex to 0.1.9, now including specific deploy instructions for Vercel.
- [x] Make sure the linting passes by running `pnpm lint`
- [x] The examples guidelines are followed from [our contributing doc](https://github.com/vercel/next.js/blob/canary/contributing.md#adding-examples)
