Added value to code example

So that it is more consistent with the tutorial text.

site/content/tutorial/02-reactivity/04-updating-arrays-and-objects/app-a/App.svelte

@@ -1,8 +1,8 @@
 <script>
 	let numbers = [1, 2, 3, 4];
 
-	function addNumber() {
-		numbers.push(numbers.length + 1);
+	function addNumber(value) {
+		numbers.push(value);
 	}
 
 	$: sum = numbers.reduce((t, n) => t + n, 0);
@@ -12,4 +12,4 @@
 
 <button on:click={addNumber}>
 	Add a number
-</button>
+</button>