Update obama_vs_mccain.Rd

[thomas6g]

Corrected description of urbanization column in obama_vs_maccain data frame.

The "Urbanization" column is currently descripted as "Percentage of people living in an urban area." which makes no sense looking at the actual data content. I checked the data source at www.census.gov and discussed the point with Edwin Thoen who provided the data set. We concluded that the "Urbanization" column contains the population per square mile.

@richierocks could you please be so kind and review the change and merge into master if appropriate.

Thank you & best regards,

Thomas