PEP 790: fix reference to future version

[JostMigenda]

Based on the annual cadence of PEP 602, two years after 3.15.0 final means 3.17.0 final. This looks like a simple typo.

• 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)

(pinging @hugovk as the PEP author & 3.15 release manager)

---

📚 Documentation preview 📚: https://pep-previews--4496.org.readthedocs.build/

[python-cla-bot]

All commit authors signed the Contributor License Agreement.

[hugovk]

Thank you!