docs: use diff codeblock for useReducer typings change (#31685)

## 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]`.


## How did you test this change?

**Before**


![2024-12-06_10-49](https://github.com/user-attachments/assets/bde94eec-a7cc-4fc8-bcca-37867633d37e)


**After**


![2024-12-06_10-51](https://github.com/user-attachments/assets/693fb7ac-68a3-45fc-aada-0c7594441b2a)

Signed-off-by: Eng Zer Jun <engzerjun@gmail.com>

CHANGELOG.md

@@ -97,11 +97,15 @@ The most common changes can be codemodded with `npx types-react-codemod@latest p
 * JSX namespace in TypeScript: The global `JSX` namespace is removed to improve interoperability with other libraries using JSX. Instead, the JSX namespace is available from the React package: `import { JSX } from 'react'`  
 * Better `useReducer` typings: Most `useReducer` usage should not require explicit type arguments.  
   For example,  
-  \-useReducer\<React.Reducer\<State, Action\>\>(reducer)  
-  \+useReducer(reducer)  
-  or  
-  \-useReducer\<React.Reducer\<State, Action\>\>(reducer)  
-  \+useReducer\<State, \[Action\]\>(reducer)
+  ```diff
+  -useReducer<React.Reducer<State, Action>>(reducer)
+  +useReducer(reducer)
+  ```
+  or
+  ```diff
+  -useReducer<React.Reducer<State, Action>>(reducer)
+  +useReducer<State, Action>(reducer)
+  ```
 
 
 ## All Changes