DOC: add a semicolon to hide long matplotlib axes return value
add a semicolon to hide debug messages like:
Out[85]: 
array([[<matplotlib.axes._subplots.AxesSubplot object at 0x7f3d0ad6e3d0>,
        <matplotlib.axes._subplots.AxesSubplot object at 0x7f3d0ad4a350>,
        <matplotlib.axes._subplots.AxesSubplot object at 0x7f3d0aa10250>,
        <matplotlib.axes._subplots.AxesSubplot object at 0x7f3d0a6367d0>],
       [<matplotlib.axes._subplots.AxesSubplot object at 0x7f3d088c2d50>,
        <matplotlib.axes._subplots.AxesSubplot object at 0x7f3d089ea310>,
        <matplotlib.axes._subplots.AxesSubplot object at 0x7f3d08d89890>,
        <matplotlib.axes._subplots.AxesSubplot object at 0x7f3d0a76ae10>],
       [<matplotlib.axes._subplots.AxesSubplot object at 0x7f3d0a756090>,
        <matplotlib.axes._subplots.AxesSubplot object at 0x7f3d0a8dd750>,
        <matplotlib.axes._subplots.AxesSubplot object at 0x7f3d096aaed0>,
        <matplotlib.axes._subplots.AxesSubplot object at 0x7f3d0937d490>],
       [<matplotlib.axes._subplots.AxesSubplot object at 0x7f3d09f49a10>,
        <matplotlib.axes._subplots.AxesSubplot object at 0x7f3d0a0551d0>,
        <matplotlib.axes._subplots.AxesSubplot object at 0x7f3d0a418550>,
        <matplotlib.axes._subplots.AxesSubplot object at 0x7f3d0a6f0ad0>]],
      dtype=object)
which you can see here:

https://pandas.pydata.org/docs/user_guide/visualization.html

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
