Init spot_size member in parking lot exercise (#83)

solutions/object_oriented_design/parking_lot/parking_lot.ipynb

@@ -71,7 +71,7 @@
     "    def __init__(self, vehicle_size, license_plate, spot_size):\n",
     "        self.vehicle_size = vehicle_size\n",
     "        self.license_plate = license_plate\n",
-    "        self.spot_size\n",
+    "        self.spot_size = spot_size\n",
     "        self.spots_taken = []\n",
     "\n",
     "    def clear_spots(self):\n",