Fix compilation error in graph_challenge notebook (donnemartin#172)

Author: gmazzola

graphs_trees/graph/graph_challenge.ipynb

@@ -124,10 +124,10 @@
     "        self.adj_weights = {}  # Key = Node, val = weight\n",
     "\n",
     "    def __repr__(self):\n",
-    "        return str(self.id)\n",
+    "        return str(self.key)\n",
     "\n",
-    "    def __lt__(self, left, right):\n",
-    "        return left.id < right.id\n",
+    "    def __lt__(self, other):\n",
+    "        return self.key < other.key\n",
     "\n",
     "    def add_neighbor(self, neighbor, weight=0):\n",
     "        # TODO: Implement me\n",