This is a quick typo fix for the Tests section of the readme file.
## Launch Checklist
<!-- Thanks for the PR! Feel free to add or remove items from the
checklist. -->
- [x] Briefly describe the changes in this PR.
- [ ] Add an entry to `CHANGELOG.md` under the `## main` section. (_so
small that this doesn't seem necessary, but will add if you would prefer
it_).
