Fix loop bug in deck of cards exercise (#396)

solutions/object_oriented_design/deck_of_cards/deck_of_cards.ipynb

@@ -122,7 +122,7 @@
     "\n",
     "    def score(self):\n",
     "        total_value = 0\n",
-    "        for card in card:\n",
+    "        for card in self.cards:\n",
     "            total_value += card.value\n",
     "        return total_value\n",
     "\n",