add commas (#633)

Author: tangciwei

src/v2/api/index.md

@@ -966,8 +966,8 @@ All lifecycle hooks automatically have their `this` context bound to the instanc
       var body   = this.$slots.default
       var footer = this.$slots.footer
       return createElement('div', [
-        createElement('header', header)
-        createElement('main', body)
+        createElement('header', header),
+        createElement('main', body),
         createElement('footer', footer)
       ])
     }