Fix code example error in create-effect.mdx (#1261)

Co-authored-by: kodiakhq[bot] <49736102+kodiakhq[bot]@users.noreply.github.com>

Author: WavJaby

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

@@ -35,7 +35,7 @@ const [a, setA] = createSignal(initialValue)
 // effect that depends on signal `a`
 createEffect((prevSum) => {
 	// do something with `a` and `prevSum`
-	const sum = a() + b()
+	const sum = a() + prevSum
 	if (sum !== prevSum) console.log("sum changed to", sum)
 	return sum
 }, 0)