docs: fix defaults in custom fetch example

[ExEr7um]

🔗 Linked issue

None.

❓ Type of change

• 📖 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

defu gives leftmost arguments the priority, so defaults should go right-side. Current solution gives defaults priority, so you can't rewrite them.

Quote from defu docs:

Leftmost arguments have more priority when assigning defaults.

📝 Checklist

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