docs(lesson): add missing "use client" directive in RSC limitations example

- Add "use client" directive to clientPage.js code block in lesson 04-E
- Clarifies client component requirements for educational accuracy

Author: ivanroguerre

lessons/04-rscs-with-nextjs/E-limitations-of-rscs.md

@@ -60,6 +60,7 @@ export default async function WhoAmI() {
 And now clientPage.js
 
 ```javascript
+"use client";
 import updateUsername from "./updateUsername";
 
 export default function ClientWhoAmIPage({ children, id }) {