docs: update markdown code block

src/switch-cell/README.md

@@ -6,7 +6,7 @@ The SwitchCell component will be deprecated in version 3.0. Please use the Cell
 
 ### Install
 
-``` javascript
+```js
 import Vue from 'vue';
 import { SwitchCell } from 'vant';
 

src/switch-cell/README.zh-CN.md

@@ -6,7 +6,7 @@
 
 ### 引入
 
-``` javascript
+```js
 import Vue from 'vue';
 import { SwitchCell } from 'vant';