improve argument name (vuejs#787)

NOTE:
  adjust to flowtype definition
  https://github.com/vuejs/vue/blob/dev/src/core/config.js#L12

Author: kazupon

src/v2/api/index.md

@@ -70,9 +70,9 @@ type: api
 - **Usage:**
 
   ``` js
-  Vue.config.errorHandler = function (err, vm, type) {
+  Vue.config.errorHandler = function (err, vm, info) {
     // handle error
-    // `type` is a Vue-specific error type, e.g. which lifecycle hook
+    // `info` is a Vue-specific error info, e.g. which lifecycle hook
     // the error was found in. Only available in 2.2.0+
   }
   ```