Update 01_dataframe.ipynb (#264)

I was looking into this and I thought either I am not fully grasping the fn or this is wrong.
Since it is using the file itself rather than the fn. so it should have been either for file in files: or then, later on, had used "fn" instead of "file", right?

Here's a link to the highlighted part: https://tutorial.dask.org/01_dataframe.html#:~:text=for%20fn%20in%20file%3A%0A%20%20%20%20df%20%3D%20pd.read_csv(os.path.join(%27data%27%2C%20%27nycflights%27%2C%20file))%0A%20%20%20%20maxes.append(df.DepDelay.max())

Author: Shadimrad

01_dataframe.ipynb

@@ -350,7 +350,7 @@
     "\n",
     "maxes = []\n",
     "\n",
-    "for fn in file:\n",
+    "for file in files:\n",
     "    df = pd.read_csv(os.path.join('data', 'nycflights', file))\n",
     "    maxes.append(df.DepDelay.max())\n",
     "    \n",