fix smoothScroll.animateScroll parameter error (vuejs#823)

"List Rendering > Array Change Detection"

> You can open the console and play with the previous examples’ items array by calling their mutation methods. For example: example1.items.push({ message: 'Baz' })

ecec `example1.items.push({ message: 'Baz' })` throw error

Author: linkHe12

src/v2/guide/list.md

@@ -49,7 +49,7 @@ var example1 = new Vue({
   },
   watch: {
     items: function () {
-      smoothScroll.animateScroll(null, '#example-1')
+      smoothScroll.animateScroll(document.querySelector('#example-1'))
     }
   }
 })
@@ -99,7 +99,7 @@ var example2 = new Vue({
   },
   watch: {
     items: function () {
-      smoothScroll.animateScroll(null, '#example-2')
+      smoothScroll.animateScroll(document.querySelector('#example-2'))
     }
   }
 })