DOC: improve description of the examplewhich dataframe has two rows

[yonashub]

Location of the documentation

[this should provide the location of the documentation, e.g. "pandas.read_csv" or the URL of the documentation, e.g. "https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.DataFrame.mode.html"]

Note: You can check the latest versions of the docs on master here.

Documentation problem

[this should provide a description of what documentation you believe needs to be fixed/improved]
... but the DataFrame has two rows. it is not clear which dataframe it refers to.

Suggested fix for documentation

[this should explain the suggested fix and why it's better than the existing documentation]
... but the resulting DataFrame has two rows.