Provide cleanup instructions on "Set up a local cluster" page

[genericmoniker]

First time experimenting with etcd, so I downloaded the binaries and started a local cluster as described here:

https://etcd.io/docs/v3.5/dev-guide/local_cluster/

For my situation I realized I needed an older version of etcd, but I don't know how to clean up the test environment from v3.5. When running v2.3.8, it fails with "experimental-v3demo cannot be disabled once it is enabled". I assume this is caused by having previously run a newer version, but I've no idea how to reset.