docs: Fixed insert incorrect example in single table inheritance docs

[tintin10q]

The .insert had the {data:...} field from vuex-orm in the examples. This pr removes that.

🔗 Linked issue

❓ Type of change

• 📖 Documentation (updates to the documentation or readme)
• 🐞 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)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

The .insert had the {data:...} field from vuex-orm in the examples. This pr removes the data field.

📝 Checklist

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