docs: fixed links to vm. (#1465)

docs/api/README.md

@@ -169,7 +169,7 @@ const store = new Vuex.Store({ ...options })
 
 -  `watch(fn: Function, callback: Function, options?: Object): Function`
 
-  Reactively watch `fn`'s return value, and call the callback when the value changes. `fn` receives the store's state as the first argument, and getters as the second argument. Accepts an optional options object that takes the same options as [Vue's `vm.$watch` method](https://vuejs.org/v2/api/#watch).
+  Reactively watch `fn`'s return value, and call the callback when the value changes. `fn` receives the store's state as the first argument, and getters as the second argument. Accepts an optional options object that takes the same options as [Vue's `vm.$watch` method](https://vuejs.org/v2/api/#vm-watch).
 
   To stop watching, call the returned unwatch function.
 

docs/zh/api/README.md

@@ -170,7 +170,7 @@ const store = new Vuex.Store({ ...options })
 
 - `watch(fn: Function, callback: Function, options?: Object): Function`
 
-  响应式地侦听 `fn` 的返回值，当值改变时调用回调函数。`fn` 接收 store 的 state 作为第一个参数，其 getter 作为第二个参数。最后接收一个可选的对象参数表示 Vue 的 [`vm.$watch`](https://cn.vuejs.org/v2/api/#watch) 方法的参数。
+  响应式地侦听 `fn` 的返回值，当值改变时调用回调函数。`fn` 接收 store 的 state 作为第一个参数，其 getter 作为第二个参数。最后接收一个可选的对象参数表示 Vue 的 [`vm.$watch`](https://cn.vuejs.org/v2/api/#vm-watch) 方法的参数。
 
   要停止侦听，调用此方法返回的函数即可停止侦听。