docs: recommend using v3 for demo

The README suggests using the latest (from feb, 2023). I'd recommend removing this reference and suggesting the latest v3/v2 which have both been validated today.

localstack-demo/README.md

Line 19 in 43f0e84

Note: Please make sure to pull and start the `latest` LocalStack Docker image. At the time of writing (2023-02-01), the demo requires some features that were only recently added to LocalStack and are not part of a tagged release version yet.