docs: use diff codeblock for useReducer typings change

[Juneezee]

Summary

This PR updates the 19.0.0 changelog to use diff-styled code blocks for illustrating changes to useReducer typings. Also removes the incorrect square brackets in the second diff, it should be Action instead of [Action].

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Name	Status	Preview	Comments	Updated (UTC)
react-compiler-playground	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Dec 6, 2024 2:58am

[react-sizebot]

Comparing: c56c623...bf8d4fd

Critical size changes

Includes critical production bundles, as well as any change greater than 2%:

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
oss-stable/react-dom/cjs/react-dom.production.js	=	6.68 kB	6.68 kB	=	1.83 kB	1.82 kB
oss-stable/react-dom/cjs/react-dom-client.production.js	=	510.81 kB	510.81 kB	=	91.43 kB	91.43 kB
oss-experimental/react-dom/cjs/react-dom.production.js	=	6.69 kB	6.69 kB	=	1.83 kB	1.83 kB
oss-experimental/react-dom/cjs/react-dom-client.production.js	=	515.74 kB	515.74 kB	=	92.14 kB	92.14 kB
facebook-www/ReactDOM-prod.classic.js	=	595.67 kB	595.67 kB	=	105.14 kB	105.14 kB
facebook-www/ReactDOM-prod.modern.js	=	585.93 kB	585.93 kB	=	103.57 kB	103.57 kB

Significant size changes

Includes any change greater than 0.2%:

(No significant changes)

Generated by 🚫 dangerJS against 1958695

[jackpope]

Thanks for the update!