docs: fix encoding; clarify example is for setup.cfg

[mwtoews]

Thanks for the docs! Here are a few minor tweaks.

Fix a few encoding values from utf=8 to utf-8.

The setuptools data file example was a bit confusing, since the default behaviour of pyproject.toml is described, but is followed by an example for setup.cfg.

Perhaps in a few years all setup.cfg examples should be either removed or replaced with standard-based pyproject.toml.

[henryiii]

Thanks!