Emphasize the importance of returning the value from createEffect to …

src/routes/reference/basic-reactivity/create-effect.mdx

@@ -37,7 +37,7 @@ createEffect((prevSum) => {
 	// do something with `a` and `prevSum`
 	const sum = a() + b()
 	if (sum !== prevSum) console.log("sum changed to", sum)
-	return sum
+	return sum // Pay attention you must return the value that will update the previous one (prevSum)
 }, 0)
 // ^ the initial value of the effect is 0
 ```