Add readability to example

[nibblerenush]

1. We get rid of the magic number 1 and thus show that the node number must be the same in MemStorage::new_with_conf_state and let cfg = Config { id: ...
2. Adding an call cfg.validate().unwrap() as it is done in the documentation https://docs.rs/raft/latest/raft/

[ti-chi-bot]

Welcome @nibblerenush! It looks like this is your first PR to tikv/raft-rs 🎉

[nibblerenush]

@BusyJay PTAL