docs: fix prompt and remove reference to tests that don't exist

[xavdid]

PR Checklist

• [ ] Addresses an existing open issue: fixes #000
• [ ] That issue was marked as accepting prs
• [x] Steps in CONTRIBUTING.md were taken

Overview

A couple of readmes reference test files that don't exist. Also, a part 2 readme used instructions from part 1, so I updated them.

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edit: also fixed instructions in readmes that specified the wrong number of type parameters and/or listed the wrong number.

[xavdid]

@JoshuaKGoldberg this one's ready whenever you have a sec to look!

[JoshuaKGoldberg]

Ah apologies, I missed this coming in!