Add const browser variable to before example

Author: heitortsergent

src/data/markdown/translated-guides/en/03 Using k6 browser/04 Migrating to k6 v0-46.md

@@ -194,6 +194,7 @@ You can open a new page by using the imported [browser](/javascript-api/k6-exper
 
 ```javascript
 export default async function () {
+    const browser = chromium.launch();
     const page = browser.newPage();
     // ...
     page.close();