format nb6 title levels

docs/source/api/notebooks/Notebook_6_API2CLI.ipynb

@@ -129,7 +129,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "### GraphStorm `GSConfig` object explanation\n",
+    "## GraphStorm `GSConfig` object explanation\n",
     "Once obtained these arguments, we can use them to create a `GSConfig` object and then pass the object to different modules to get related configurations. The `GSConfig` object can check all arguments' format and values to ensure compliance with GraphStorm specifications. Below cells show the code of creating the `GSConfig` object and examples of how to use it. Please refer to the [GSConfig API doc](https://graphstorm.readthedocs.io/en/latest/api/generated/graphstorm.config.GSConfig.html#graphstorm.config.GSConfig) for more details of this class."
    ]
   },