Relabel y-axis as "frequency" in the bootstrapping plots

[tompollard]

In https://github.com/carpentries-incubator/machine-learning-novice-python/blob/gh-pages/_episodes/07-bootstrapping.md, the page includes two "density" plots.

The y-axis should be probably be relabelled as "Frequency" because the range shows an absolute count of data points.