add bins argument to Histogram function
The primary argument used when plotting histograms is `bins`. This argument is not in the pandas documentation so you do not see it when auto-tabbing for documentation. I have added  `bins=10` as an argument to the dataframe and series histogram function while also adding a line of documentation. `10` is the matplotlib default bin value so this is only making `bins` visible, not adding or changing any plotting functionality.
