[GraphBolt] update OnDiskDataset tutorial (#6783)

notebooks/stochastic_training/ondisk_dataset_homograph.ipynb

@@ -105,7 +105,7 @@
         "For homogeneous graph, we just need to save edges(namely node pairs) into **CSV** file.\n",
         "\n",
         "Note:\n",
-        "when saving to file, do not save index and header.*italicized text*\n"
+        "when saving to file, do not save index and header.\n"
       ],
       "metadata": {
         "id": "qhNtIn_xhlnl"
@@ -457,4 +457,4 @@
       "outputs": []
     }
   ]
-}
\ No newline at end of file
+}