add small comment to explain why we store local csv

module3/exo1/analyse-syndrome-grippal.ipynb

@@ -38,6 +38,13 @@
     "data_url = \"http://www.sentiweb.fr/datasets/incidence-PAY-3.csv\""
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "We download the file to make sure this notebook is working even if url changed in the future."
+   ]
+  },
   {
    "cell_type": "code",
    "execution_count": 25,