PEP 635: Minor typo fix in code sample.

[Mariatta]

Looks like an unclosed f-string.

• Change is either:
  • [ ] To a Draft PEP
  • [ ] To an Accepted or Final PEP, with Steering Council approval
  • [x] To fix an editorial issue (markup, typo, link, header, etc)
• [x] PR title prefixed with PEP number (e.g. PEP 123: Summary of changes)

---

📚 Documentation preview 📚: https://pep-previews--3871.org.readthedocs.build/