hist2D requires capitalization of the "D", but axis.hist2d needs to be lowercase

commit 40632c7 explains that in PyPlot the "D"s in `hist2D` and the like are capitalized in PyPlot.
The `pyplot wrapper` requires capitalization, but when the function is called on/by an axis object, the opposite is true:
```
hist2D(...) # works
hist2d(...) # throws ERROR: UndefVarError: `hist2d` not defined in `Main`
fig, ax = plt.sublots()
ax.hist2D(...) # throws ERROR: KeyError: key :hist2D not found
ax.hist2d(...) # works
```