etcdctl: Fix migrate example in README.md
@@ -1214,7 +1214,7 @@ The provided transformer should read until EOF and flush the stdout before exiti
#### Example
```
-./etcdctl --data-dir=/var/etcd --transformer=k8s-transformer
+./etcdctl migrate --data-dir=/var/etcd --transformer=k8s-transformer
# finished transforming keys
```
