Corrected path length calculation.

Per @Cilph's suggestion, I replaced the summed cost over all nodes along a path with the cost of the destination node. Some quick testing in Kerbin orbit confirms that communication times behave more consistently than reported by @bk2w at Cilph/RemoteTech2Archived#219.

 Fixed #11.

EDIT: GAH, I give up on how to close an issue from a pull request.