A bug of notebook format (#164)

It's JSON. just missing a comma.

notebooks/4_Utils/tensorboard_basic.ipynb

@@ -176,7 +176,7 @@
   },
   {
    "cell_type": "markdown",
-   "metadata": {}
+   "metadata": {},
    "source": [
     "### Loss and Accuracy Visualization\n",
     "\n",