incorrect default cluster mode

Cluserting documentation says that the default for `--cluster_mode` is 'emulated' if not specified. However the implementation suggests this is not the case.

https://github.com/dragonflydb/dragonfly/blob/be11fa02cd0721aab2a3cbf7d23fb84f1f51d432/src/server/cluster_support.cc#L60-L69
```
if (cluster_mode_str == "emulated") {
cluster_mode = ClusterMode::kEmulatedCluster;
} else if (cluster_mode_str == "yes") {
cluster_mode = ClusterMode::kRealCluster;
} else if (cluster_mode_str.empty()) {
cluster_mode = ClusterMode::kNoCluster;
} else {
LOG(ERROR) << "Invalid value for flag --cluster_mode. Exiting...";
exit(1);
}
```