Minor fixes in @auth/express documentation

[waigel]

☕️ Reasoning

• fix wrong reference link to core/providers
• fix missing async keyword in express authorization middleware example

🧢 Checklist

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

🎫 Affected issues

[vercel[bot]]

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Name	Status	Preview	Comments	Updated (UTC)
auth-docs	❌ Failed (Inspect)			Feb 24, 2024 5:37pm

1 Ignored Deployment

Name	Status	Preview	Comments	Updated (UTC)
next-auth-docs	⬜️ Ignored (Inspect)	Visit Preview		Feb 24, 2024 5:37pm

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