Update state-management.md #2 (vuejs#2488)

use `var` consistently. previous PR was using `const`, it should be using `var`. ref: vuejs#2486 (comment)

src/v2/guide/state-management.md

@@ -19,13 +19,13 @@ If you're coming from React, you may be wondering how vuex compares to [redux](h
 It is often overlooked that the source of truth in Vue applications is the raw `data` object - a Vue instance only proxies access to it. Therefore, if you have a piece of state that should be shared by multiple instances, you can share it by identity:
 
 ``` js
-const sourceOfTruth = {}
+var sourceOfTruth = {}
 
-const vmA = new Vue({
+var vmA = new Vue({
   data: sourceOfTruth
 })
 
-const vmB = new Vue({
+var vmB = new Vue({
   data: sourceOfTruth
 })
 ```