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>