[docs] fix inconsistent margin in svelte/motion spring tutorial (#8081)

Author: cubedhuang

site/content/tutorial/09-motion/02-spring/app-b/App.svelte

@@ -33,6 +33,7 @@
 	svg {
 		width: 100%;
 		height: 100%;
+		margin: -8px;
 	}
 	circle {
 		fill: #ff3e00;