Update example for calculating fullName from state

Clarify the example of updating state based on props or state in the documentation.

Author: Sekhard26

src/content/learn/you-might-not-need-an-effect.md

@@ -34,6 +34,7 @@ To help you gain the right intuition, let's look at some common concrete example
 
 Suppose you have a component with two state variables: `firstName` and `lastName`. You want to calculate a `fullName` from them by concatenating them. Moreover, you'd like `fullName` to update whenever `firstName` or `lastName` change. Your first instinct might be to add a `fullName` state variable and update it in an Effect:
 
+```js {expectedErrors: {'react-compiler': [8]}} {5-9}
 ```js {5-9}
 function Form() {
   const [firstName, setFirstName] = useState('Taylor');