docs: Fix typo in custom-pages.md

[bahulneel]

The error page example has a typo in the script section.

🔗 Linked issue

https://github.com/sidebase/nuxt-auth/issues/893

❓ 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

📝 Checklist

• [x] I have linked an issue or discussion.
• [x] I have added tests (if possible).
• [x] I have updated the documentation accordingly.

[pkg-pr-new[bot]]

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pnpm add https://pkg.pr.new/@sidebase/nuxt-auth@894

commit: 2792bb8