Added example of the legend keyword in kdeplot.

I added an example demonstrating how to use the legend keyword with
kdeplot. I also checked all of the distplot examples to make sure that
there are no side effects. I also tested the example at cell 21 ("To
control any of the underlying plots, pass keyword arguments to the
[plot]_kws argument.") using values of True and False for the displot
legend keyword. No problems were observed, and I set it back to False
to match the original notebook.