Fix minor typo in assets docs

docs/guide/structure-assets.md

@@ -240,7 +240,8 @@ be the corresponding [configuration arrays](concept-configurations.md).
 > Tip: You can conditionally choose which assets to use in an asset bundle. The following example shows how
 > to use `jquery.js` in the development environment and `jquery.min.js` otherwise:
 >
->  ```php
+
+```php
 'yii\web\JqueryAsset' => [
     'js' => [
         YII_ENV_DEV ? 'jquery.js' : 'jquery.min.js'