PEP 751: Some minor copy edits

[pfmoore]

@brettcannon This is just a couple of typo fixes for PEP 751 that I spotted while reviewing.

There are only two that are significant:

1. I added a note that tools could choose to only support a subset of VCS implementations. I think this is basically self-evident (we wouldn't want to require everyone to support Subversion) but it's worth being explicit.
2. I removed an incomplete comment in "How to teach this". I couldn't work out what you wanted to say, but feel free to put back the complete version if you think it's worth having 🙂

The rest are just typos.

Now off to review the Discourse threads. I could be some time 🙂

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📚 Documentation preview 📚: https://pep-previews--4313.org.readthedocs.build/

[brettcannon]

Thanks!