PEP 831: Fix typo in JIT frame pointer explanation

[brettcannon]

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

---

📚 Documentation preview 📚: https://pep-previews--4919.org.readthedocs.build/

[savannahostrowski]

Thanks Brett!