Update refs.mdx

Signal value should be used as red, not the signal setter.

Author: alexmuzenhardt

src/routes/concepts/refs.mdx

@@ -86,7 +86,7 @@ function App() {
 			<button onClick={() => setShow((isShown) => !isShown)}>Toggle</button>
 
 			<Show when={show()}>
-				<p ref={setElement}>This is the ref element</p>
+				<p ref={element}>This is the ref element</p>
 			</Show>
 		</div>
 	)