Currently the `etcdctl ls` subcommand help output is a bit misleading.
It mentions that using the `--recursive` flag will output all keys and
values for a given path:
--recursive returns all values for key and child keys
This is inaccurate. The `--recursive` will only output the key names
under the given path. Fix the issue by updating the help string for
the `--recursive` flag.
--recursive returns all key names recursively for the given path
Fixes#2379.
