Update convolutional_network_raw.ipynb (#366)

tensorflow_v2/notebooks/3_NeuralNetworks/convolutional_network_raw.ipynb

@@ -217,7 +217,7 @@
     "        loss = cross_entropy(pred, y)\n",
     "        \n",
     "    # Variables to update, i.e. trainable variables.\n",
-    "    trainable_variables = weights.values() + biases.values()\n",
+    "    trainable_variables = list(weights.values()) + list(biases.values())\n",
     "\n",
     "    # Compute gradients.\n",
     "    gradients = g.gradient(loss, trainable_variables)\n",