update image processing example notebok imports and function call

[RRosio]

I was running into the error:

AttributeError: module 'matplotlib' has no attribute 'image'

and I found two workarounds, using import matplotlib.pyplot as plt to use the pyplot submodule's imsave() or to import the image submodule with import matplotlib.image as mpimg to use its imsave() function. After some brief research I could not discern any major differences between the two functions so I have gone with the pyplot submodule's function.

[github-actions]

👈 Launch a binder notebook on branch RRosio/ipywidgets/nb-image-proc

[martinRenou]

Thanks!