docs: fix `compatibilityDate` value

[yamanoku]

❓ Type of change

• [x] 📖 Documentation (updates to the documentation, readme, or JSdoc annotations)
• [ ] 🐞 Bug fix (a non-breaking change that fixes an issue)
• [ ] 👌 Enhancement (improving an existing functionality like performance)
• [ ] ✨ New feature (a non-breaking change that adds functionality)
• [ ] 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• [ ] ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

The compatibilityDate value uses the DateString type and should be in YYYY--MM-DD format, but the documentation incorrectly described it as YY-MM-DD, so I've corrected it.

📝 Checklist

• [ ] I have linked an issue or discussion.
• [x] I have updated the documentation accordingly.

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[pi0]

Thanks for PR. Can you please rebase your commit against v2 branch

[yamanoku]

@pi0 I’ve rebased and switched it to the v2 branch. Please review.