fix code in comment

Author: heinzK1X

Chapter1_Introduction/Chapter1_Introduction.ipynb

@@ -171,7 +171,7 @@
       "    2. Also in the styles is  bmh_matplotlibrc.json file. This can be used to\n",
       "       update the styles in only this notebook. Try running the following code:\n",
       "\n",
-      "        import json\n",
+      "        import json, matplotlib\n",
       "        s = json.load( open(\"../styles/bmh_matplotlibrc.json\") )\n",
       "        matplotlib.rcParams.update(s)\n",
       "\n",
@@ -1098,4 +1098,4 @@
    "metadata": {}
   }
  ]
-}
+}