Fix dispatch_cfg_t comment

  struct smartlist_t *type_by_msg;

  /** A list of channel_id_t (cast to void*), indexed by msg_t. */

  struct smartlist_t *chan_by_msg;

  /** A list of dispatch_rcv_t, indexed by msg_type_id_t. */

  /** A list of dispatch_typefns_t, indexed by msg_type_id_t. */

  struct smartlist_t *fns_by_type;

  /** A list of dispatch_typefns_t, indexed by msg_t. */

  /** A list of dispatch_rcv_t, indexed by msg_t. */

  struct smartlist_t *recv_by_msg;

};