chore(docs): Fixes refresh token example

[jakehewitt]

☕️ Reasoning

What changes are being made? What feature/bug is being fixed here?

• On initial login account.expires_at is a timestamp and shouldn't be added to Date.now().
• When refreshing the token refreshedTokens.expires_at is undefined. It should be using refreshedTokens.expires_in

🧢 Checklist

• [X] Documentation
• [ ] Tests
• [X] Ready to be merged

🎫 Affected issues

Please scout and link issues that might be solved by this PR.

Fixes: No open issues

Related to: https://github.com/nextauthjs/next-auth/pull/4540

📌 Resources

• Contributing guidelines
• Code of conduct
• Contributing to Open Source

[vercel[bot]]

The latest updates on your projects. Learn more about Vercel for Git ↗︎

Name	Status	Preview	Updated
next-auth	✅ Ready (Inspect)	Visit Preview	Jul 6, 2022 at 4:29PM (UTC)

[vwatel]

Any updates on this one? We are currently looking at using this library, which looks very solid and clean. We miss the refresh token rotation at the moment to have it fully integrated with the library.

[balazsorban44]

This documentation has been rewritten at: https://authjs.dev/guides/basics/refresh-token-rotation

The related issue about expires_in vs expires_at is tracked here #6538