docs: update markdown code block

src/sidebar/README.md

@@ -2,7 +2,7 @@
 
 ### Install
 
-``` javascript
+```js
 import Vue from 'vue';
 import { Sidebar, SidebarItem } from 'vant';
 
@@ -22,7 +22,7 @@ Vue.use(SidebarItem);
 </van-sidebar>
 ```
 
-``` javascript
+```js
 export default {
   data() {
     return {

src/sidebar/README.zh-CN.md

@@ -2,7 +2,7 @@
 
 ### 引入
 
-``` javascript
+```js
 import Vue from 'vue';
 import { Sidebar, SidebarItem } from 'vant';
 
@@ -24,7 +24,7 @@ Vue.use(SidebarItem);
 </van-sidebar>
 ```
 
-``` javascript
+```js
 export default {
   data() {
     return {