docs: include mutations in the Vuex 4.x example (#1868)

Author: kiaking

docs/guide/README.md

@@ -47,6 +47,11 @@ const store = createStore({
     return {
       count: 1
     }
+  },
+  mutations: {
+    increment (state) {
+      state.count++
+    }
   }
 })