Updated lesson3-planet notebook (#94)
Updated lesson3-planet notebook (#94)

Here `path` is `/home/jupyter/.fastai/data/planet`.
Recall that the default behavior of `Path.mkdir` function will raise `FileNotFoundError` when there is a missing parent.
In a freshly installed new instance, there will not be a `data` directory under `.fastai` directory. Thus, the current command will raise `FileNotFoundError`.

Here is what the Python will raise:
`FileNotFoundError: [Errno 2] No such file or directory: '/home/jupyter/.fastai/data/planet'`

After adding an argument `parents=True`, any missing parents of this path are created as needed, which will produce the desired result in a new instance.
fastai/course-v3,fastai/course-v3,fastai/course-v3,fastai/course-v3,fastai/course-v3,fastai/course-v3,fastai/course-v3