Update button.mdx

[creeponsky]

some FButton's icon -> child

Describe the changes

Checklist

• [ ] I have read the CONTRIBUTING.md.
• [ ] I have included the relevant unit/golden tests.
• [ ] I have included the relevant samples.
• [ ] I have updated the documentation accordingly.
• [ ] I have updated the CHANGELOG.md if necessary.

[auto-maid[bot]]

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[Pante]

Thanks for the fixes!