DOC: add semicolons to suppress text repr of matplotlib objects in visualization.rst
add semicolons (";") after the plotting function to suppress unnecessary debug messages similar to:

<matplotlib.axes._subplots.AxesSubplot at 0x7fe278bb5160>

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