docs: add missing https protocol to Stately Studio link (#5585)

The Stately Studio CTA link in the root README and the xstate package
README was written without a scheme (stately.ai/...), so GitHub and npm
rendered it as a relative URL pointing at
github.com/statelyai/xstate/blob/main/stately.ai/... instead of the
Stately registry. Add the missing https:// to match the correct link
used a few lines below.