Update render-function.md (#708)

`Array.apply(null, { length: n })` is too hard to understand.
`Array.from({ length: n })` is much more straightforward in given context. Arrow functions are used in examples, so we can use ES2015.

Author: exarus

src/guide/render-function.md

@@ -233,7 +233,7 @@ If you really want to duplicate the same element/component many times, you can d
 ```js
 render() {
   return Vue.h('div',
-    Array.apply(null, { length: 20 }).map(() => {
+    Array.from({ length: 20 }).map(() => {
       return Vue.h('p', 'hi')
     })
   )