Update filters, global filters needs to go before Vue instance creation (#1392)

Global filters defined after creating the Vue instance throws `Failed to resolve filter`. Reference https://forum.vuejs.org/t/global-filters-failing-to-resolve-inside-single-file-components/21863/6

Author: brandedoutcast

src/v2/guide/filters.md

@@ -26,14 +26,18 @@ filters: {
 }
 ```
 
-或者全局定义过滤器：
+或者在创建 Vue 实例之前全局定义过滤器：
 
 ``` js
 Vue.filter('capitalize', function (value) {
   if (!value) return ''
   value = value.toString()
   return value.charAt(0).toUpperCase() + value.slice(1)
 })
+
+new Vue({
+  // ...
+})
 ```
 
 下面这个例子用到了 `capitalize` 过滤器：