message: Update graph.h
  change comment for  Graph::FindEdgeId method.
diff: @@ -617,9 +617,9 @@ class Graph {
  int num_edge_ids() const { return edges_.size(); }
  // Returns the Edge associated with an id, or nullptr if no edge
  - // with that id (the node with that id was removed and the id has
  + // with that id (the edge with that id was removed and the id has
  // not yet been re-used). *this owns the returned instance.
  - // REQUIRES: 0 <= id < num_node_ids().
  + // REQUIRES: 0 <= id < num_edge_ids().
  const Edge* FindEdgeId(int id) const { return edges_[id]; }
  // Access to the set of all edges. Example usage:
  