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
```

比较 issue 中演示的 hist2D/hist2d 调用所对应的 pyplot wrapper 和轴对象处理。 在 PyPlot.jl 中复现这两种形式，然后跟踪 wrapper 和轴查找路径，以确定大小写不匹配的位置。当 pyplot 调用和轴对象调用都能按预期拼写正常工作时，即视为完成。

由索引模型根据 Issue 内容生成。