Add tox config example

[umonaca]

When I set the following config in tox.ini

[isort]
profile = "black"
multi_line_output = 3

I am getting the following error when running tox on py39:

  py39 run-test: commands[1] | poetry run isort obplatform -c
  Traceback (most recent call last):
    File "/home/runner/work/obplatform/obplatform/.tox/py39/bin/isort", line 8, in <module>
      sys.exit(main())
    File "/home/runner/work/obplatform/obplatform/.tox/py39/lib/python3.9/site-packages/isort/main.py", line 1108, in main
      config = Config(**config_dict)
    File "/home/runner/work/obplatform/obplatform/.tox/py39/lib/python3.9/site-packages/isort/settings.py", line 365, in __init__
      raise ProfileDoesNotExist(profile_name)
  isort.exceptions.ProfileDoesNotExist: Specified profile of "black" does not exist. Available profiles: black,django,pycharm,google,open_stack,plone,attrs,hug,wemake,appnexus.

I fixed it by removing the quotes around "black":

[isort]
profile = black
multi_line_output = 3

I think this example could be useful in the documentation.