docs: fix blockquote language (#3205)

Author: ocmrz

src/guide/built-ins/transition.md

@@ -330,7 +330,7 @@ In comparison, properties like `height` or `margin` will trigger CSS layout, so
 
 You can hook into the transition process with JavaScript by listening to events on the `<Transition>` component:
 
-```html
+```vue-html
 <Transition
   @before-enter="onBeforeEnter"
   @enter="onEnter"