Assignment #3 (Testing): README has unnecessary instruction

In the 3rd step of section "2- Fixtures" of the 3rd Assignment, it says to "modify your main function so that the cleaned DataFrame is always returned. That will allow you to compare it with the expected fixture".
As we will be testing each function (load_data, clean_data and save_data) individually, it does not make sense to be testing the main function as well. This means that there is no point in making main return the cleaned dataframe (the cleaned dataframe is already returned by the cleaned_data function).