docs: fix demo locale

example/index.html

@@ -12,6 +12,7 @@
     <div id="app"></div>
     <script src="https://unpkg.com/vue@2.6.10/dist/vue.js"></script>
     <script src="https://unpkg.com/vue2-datepicker/index.min.js"></script>
+    <script src="https://unpkg.com/vue2-datepicker/locale/zh-cn.js"></script>
     <script src="./index.js"></script>
   </body>
 </html>

example/index.js

@@ -1,5 +1,7 @@
 import App from './app';
 
+window.DatePicker.locale('en');
+
 new window.Vue({
   render: h =>
     h(App, {