The instruction of the README in the root directory is out of date

- [x] I have searched the [issues](https://github.com/apache/dubbo/issues) of this repository and believe that this is not a duplicate.

## Describe the proposal

The instruction was written at the time when the dubble-samples-api runs with multicast registry. But now it requires zookeeper registry started first. We should update the instruction to give new comer a better first impression.

Start with the README in the repository root and find the instructions for running dubble-samples-api. Compare the outdated multicast-registry guidance with the current requirement to start a zookeeper registry first. Done means the README accurately explains the required startup order and gives newcomers a correct first-run experience.

Written by the indexing model from the issue text.