fix typo (#394)

object spread syntax is a stage-3 proposal

docs/en/mutations.md

@@ -103,7 +103,7 @@ Since a Vuex store's state is made reactive by Vue, when we mutate the state, Vu
 
   - Use `Vue.set(obj, 'newProp', 123)`, or -
 
-  - Replace that Object with a fresh one. For example, using the stage-2 [object spread syntax](https://github.com/sebmarkbage/ecmascript-rest-spread) we can write it like this:
+  - Replace that Object with a fresh one. For example, using the stage-3 [object spread syntax](https://github.com/sebmarkbage/ecmascript-rest-spread) we can write it like this:
 
     ``` js
     state.obj = { ...state.obj, newProp: 123 }