Include where `setState` is comming from (#3652)

Co-authored-by: Sebastian Silbermann <silbermann.sebastian@gmail.com>

content/docs/hooks-reference.md

@@ -76,6 +76,7 @@ If your update function returns the exact same value as the current state, the s
 > Unlike the `setState` method found in class components, `useState` does not automatically merge update objects. You can replicate this behavior by combining the function updater form with object spread syntax:
 >
 > ```js
+> const [state, setState] = useState({});
 > setState(prevState => {
 >   // Object.assign would also work
 >   return {...prevState, ...updatedValues};