docs: tiny update on example message-ports.md (#39884)

Update message-ports.md

fix multiplying object * a number. It would multiply number * number.

docs/tutorial/message-ports.md

@@ -365,7 +365,7 @@ window.onmessage = (event) => {
     // process.
     port.onmessage = (event) => {
       console.log('from main process:', event.data)
-      port.postMessage(event.data * 2)
+      port.postMessage(event.data.test * 2)
     }
   }
 }