docs/fix-lightning-css-typo-postcss-readme (#19913)

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## Summary
Fix a typo in the @tailwindcss/postcss README by changing Lighting CSS
to Lightning CSS in the optimize option documentation.
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## Test plan
Verified the README now says Lightning CSS and that the diff is
docs-only.
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packages/@tailwindcss-postcss/README.md

@@ -110,7 +110,7 @@ export default {
 }
 ```
 
-You may also pass options to `optimize` to enable Lighting CSS but prevent minification:
+You may also pass options to `optimize` to enable Lightning CSS but prevent minification:
 
 ```js
 import tailwindcss from '@tailwindcss/postcss'