fix consistency between "data frame" and "dataframe" #17

The book uses both "data frame(s)" and "dataframe(s)" for non-object type (non-DataFrame) references.

• "data frame" example: https://aeturrell.github.io/python4DS/exploratory-data-analysis.html#skimpy-for-summary-statistics
• "dataframe" example: https://aeturrell.github.io/python4DS/exploratory-data-analysis.html#id2

I can create a pull request to convert all "dataframe(s)" into "data frame(s) excluding DataFrame, if it is plausible.