Explicitly mention which file to edit in challenge 'Use classes to abstract out data loading'

[svenvanderburg]

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In challenge 'Use classes to abstract out data loading' in episode 3.3 I suggest to change the last sentence:

Finally, construct an instance of the class CSVDataSource outside the statistical analysis and pass it to analyse_data() function.'

to

Finally, construct an instance of the class CSVDataSource outside the statistical analysis and pass it to analyse_data() function in the file inflammation-analysis.py.

Our participants were struggling with this, see #395