watch for update vue code example

Example: vuejs set

Vue.set(vm.someObject, 'propertyName', value)
// Or using alias
this.$set(this.someObject, 'propertyName', value)
// For an array, simply repalce propertyName with the index
this.$set(this.someArray, indexOfItem, value)
// Or assign new props to an object
this.someObject = Object.assign({}, this.someObject, { a: 1, b: 2 })