docs(postcss): remove duplicated optimize example from README (#19938)

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## Summary

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your change solve the problem?

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Removed a duplicated optimize: { minify: false } example from
@tailwindcss/postcss README (doc-only, one file).

## Test plan

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Check the duplicate block is removed, the earlier optimize example still
exists, and only packages/@tailwindcss-postcss/README.md changed.