Miscellaneous minor wording / emphasis / link changes

[mfisher87]

If this pull request addresses an open issue on the repository, please add 'Closes #NN' below, where NN is the issue number.

Please briefly summarise the changes made in the pull request, and the reason(s) for making these changes.

I completed instructor training recently (it was awesome) and took some notes where I got briefly "stuck" and distracted reading the curriculum. More rationale in commit messages about the specific changes :)

As I was working on this I also noticed some links in CONTRIBUTING.md didn't behave as expected, and looking at CONTRIBUTING.md in other lesson repos, I think the preferred terminology changed from "master repository" to "upstream repository", so I got that while I was in there too.

Thanks all!

If any relevant discussions have taken place elsewhere, please provide links to these.

For more guidance on how to contribute changes to a Carpentries project, please review [the Contributing Guide](CONTRIBUTING.md) and [Code of Conduct](https://docs.carpentries.org/topic_folders/policies/code-of-conduct.html). Please keep in mind that lesson Maintainers are volunteers and it may be some time before they can respond to your contribution. Although not all contributions can be incorporated into the lesson materials, we appreciate your time and effort to improve the curriculum. If you have any questions about the lesson maintenance process or would like to volunteer your time as a contribution reviewer, please contact The Carpentries Team at team@carpentries.org.

[github-actions[bot]]

:ok: Pre-flight checks passed :smiley:

This pull request has been checked and contains no modified workflow files, spoofing, or invalid commits.

It should be safe to Approve and Run the workflows that need maintainer approval.

[ndporter]

@tobyhodges is this still relevant? If so, can you review and approve or request changes? Thanks.