Mini fix in rootState example. (vuejs#629)

* Mini fix in rootState example.

Maybe I misunderstood the example, but without parenthesis the `%` operator will be executed before the `+`.

* Update modules.md

Author: agustin-tiendanube

docs/en/modules.md

@@ -58,7 +58,7 @@ const moduleA = {
   // ...
   actions: {
     incrementIfOddOnRootSum ({ state, commit, rootState }) {
-      if (state.count + rootState.count % 2 === 1) {
+      if ((state.count + rootState.count) % 2 === 1) {
         commit('increment')
       }
     }