chore: textfield prefix design feedback docs examples

[snowystinger]

Closes

Design had feedback on the prefix examples for TextField and NumberField that they weren't realistic enough. This makes them more like how they envisioned it being used.

✅ Pull Request Checklist:

• Included link to corresponding React Spectrum GitHub Issue.
• Added/updated unit tests and storybook for this change (for new code or code which already has tests).
• Filled out test instructions.
• Updated documentation (if it already exists for this component).
• Looked at the Accessibility Practices for this feature - Aria Practices

📝 Test Instructions:

Docs examples weren't realistic enough and looked funny as a result. Go to TextField and NumberField and check that these looks reasonable.

🧢 Your Project:

[rspbot]

Build successful! 🎉

• View the storybook
• View the S2 storybook
• View the S2 docs
• View the React Aria docs