Fix code typo in docs (#470)

Author: rafiqrahim

docs/en/actions.md

@@ -76,7 +76,7 @@ A more practical example of real-world actions would be an action to checkout a
 
 ``` js
 actions: {
-  checkout ({ commit, state }, payload) {
+  checkout ({ commit, state }, products) {
     // save the items currently in the cart
     const savedCartItems = [...state.cart.added]
     // send out checkout request, and optimistically

docs/fr/actions.md

@@ -76,7 +76,7 @@ Un exemple plus pratique d'une application du monde réel serait une action pour
 
 ``` js
 actions: {
-  checkout ({ commit, state }, payload) {
+  checkout ({ commit, state }, products) {
     // sauvegarder les articles actuellement dans le panier
     const savedCartItems = [...state.cart.added]
     // envoyer la requête de checkout, et vider le panier

docs/ru/actions.md

@@ -76,7 +76,7 @@ store.dispatch({
 
 ``` js
 actions: {
-  checkout ({ commit, state }, payload) {
+  checkout ({ commit, state }, products) {
     // сохраним находящиеся на данный момент в корзине товары
     const savedCartItems = [...state.cart.added]
     // инициируем запрос и "оптимистично" очистим корзину

docs/zh-cn/actions.md

@@ -76,7 +76,7 @@ store.dispatch({
 
 ``` js
 actions: {
-  checkout ({ commit, state }, payload) {
+  checkout ({ commit, state }, products) {
     // 把当前购物车的物品备份起来
     const savedCartItems = [...state.cart.added]
     // 发出结账请求，然后乐观地清空购物车