React computed state example should use `useMemo` hook

[Lsnsh]

https://react.dev/reference/react/useMemo

https://vuejs.org/guide/essentials/computed.html#computed-caching-vs-methods

In actual coding, if only have some easy calculate, use expression is better, because hook also have expenses

• https://react.dev/reference/react/useMemo#how-to-tell-if-a-calculation-is-expensive
• https://react.dev/reference/react/useMemo#should-you-add-usememo-everywhere

[matschik]

Duplicate #120

[matschik]

Other examples don't use memoization. If you want useMemo to be featured, please check out this issue: #122
In the computed snippet, using memoization makes no sense. It's over optimization.