Point documentation to http: instead of https:

The certificate at that URL is for `readthedocs.io`, so if you want to use https, it's best to use `flake8.readthedocs.io` and not `flake8.pycqa.org`.  As it is currently, you get a certificate error.

Note also that the `Code of Conduct` and `Getting Started Contributing` links are using `http` and pointing to the insecure domain, so if you do want https, you should change those as well.