docs: clarify comment

src/main.ts

@@ -7,7 +7,8 @@ const element = document.querySelector("p");
 let current = BigInt(2);
 
 for (let round = 0; element && round < 20; ++round) {
-    // worker.obj is a proxy for Calculator
+    // worker.obj is a proxy for the Calculator object on the worker
+    // thread
     current = await worker.obj.multiply(current, current);
     element.textContent = `${current}`;
 }