Fixed bind:checked tutorial example

Clicking the checkbox wasn't changing the text properly at https://svelte.dev/tutorial/checkbox-inputs. It seems like the "bind:" got left out somehow

site/content/tutorial/06-bindings/03-checkbox-inputs/app-a/App.svelte

@@ -3,7 +3,7 @@
 </script>
 
 <label>
-	<input type=checkbox checked={yes}>
+	<input type=checkbox bind:checked={yes}>
 	Yes! Send me regular email spam
 </label>
 
@@ -15,4 +15,4 @@
 
 <button disabled={!yes}>
 	Subscribe
-</button>
+</button>