Update ex09 notebook to use different axis selector

[agstephens]

For Task 5, instead of using:

daily_max = ds.t2m.max(axis=0)

you can also use

daily_max = ds.t2m.max(dim='time')

taking advantage of xarray's labelling of dimensions.

Update the notebook accordingly, but keep in the old version commented out - so as to avoid confusing the video.