DOC: Fix sparse and dense array memory usage comparison.
This MR fixes misleading part of sparse user guide, which suggests that dataframes consume far less memory than they really do, due to wrong units calculation.

Also changed usage of `str.format()` to more modern f-string syntax in part of guide which was mentioned above.
/preview Website preview of this PR available at: https://pandas.pydata.org/preview/pandas-dev/pandas/61495/