R stacked histogram example is of type "overlay" instead of "stack"

[bernardbeckerman]

In /documentation/_posts/r/statistical/2015-07-30-histograms.Rmd, the example stacked histogram contains barmode = "overlay" instead of barmode = "stack" (Line 137) yielding an example that has an overlaid rather than a stacked histogram (https://plot.ly/r/histograms/#stacked-histograms).

[I was able to make the fix locally but was having trouble installing nokogiri on my local machine to preview the changes.]