Fix pub namespace for cmd_vel_mux to keep backwards compatibility

`nh` was previously a private node handle. In my latest commit (https://github.com/yujinrobot/yujin_ocs/commit/441f6b80ba17fc990af66b17f2aa27c0d571fb14) I changed the naming (`nh` -> normale node handle, `nh_priv` -> private node handle) and updated the subs and pubs to keep the topic namespaces as before. However, I just realised I missed on publisher.

The commit in this PR fixes that.

@jihoonl please confirm and also merge into indigo. Thx!