Update api.md (#998)

docs/en/api.md

@@ -34,11 +34,12 @@ const store = new Vuex.Store({ ...options })
 
     ``` js
     {
-      state,     // same as store.state, or local state if in modules
-      rootState, // same as store.state, only in modules
-      commit,    // same as store.commit
-      dispatch,  // same as store.dispatch
-      getters    // same as store.getters
+      state,      // same as store.state, or local state if in modules
+      rootState,  // same as store.state, only in modules
+      commit,     // same as store.commit
+      dispatch,   // same as store.dispatch
+      getters,    // same as store.getters, or local getters if in modules
+      rootGetters // same as store.getters, only in modules
     }
     ```