Avoid render error due to incomplete code

[SultanOrazbayev]

The sample code in the exercise in the docs contains an 'error' when rendered on the website. The problem entry is this:

complete the following:

fig = plt.figure(figsize=(16, 8)) plt.imshow(..., cmap='RdBu_r')

As I understand, this is meant to be there as a fill-in exercise to be completed during tutorial, but to avoid the error in the rendered version, perhaps it can placed in a markdown block... (there might be a better way to deal with this, e.g. marking some cells to be ignored during execution, but since it's just one cell here, maybe that would be excessive)

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[SultanOrazbayev]

Apologies for re-formatting, I ran the code through isort and black.

[pavithraes]

@SultanOrazbayev Nice catch, and thank you for this PR!