Update Select.mdx

[rafighajili]

added renderValue prop for rendering custom ReactNode inside select button

Closes

✅ 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:

🧢 Your Project:

[snowystinger]

No need to add a prop for renderValue, this would be taken care of like so

<SelectValue>
  {({defaultChildren}) => 
    return defaultChildren; // do any context around this which can be used to pass values to the ListBoxItem and tell it other ways to render
    // this will already handle the props.placeholder and a default placeholder
  }
</SelectValue>

I'm going to close this PR in favor of #6425
Feel free to open an issue if there's anything missed

[snowystinger]

Closing in favor of #6425