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

In challenge ['Use classes to abstract out data loading'](https://carpentries-incubator.github.io/python-intermediate-development/33-code-decoupling-abstractions/index.html#exercise-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