回调函数格式优化

Signed-off-by: Nwangkai <wangkai424@huawei.com>

zh-cn/application-dev/reference/apis/js-apis-distributedKVStore.md

@@ -3867,7 +3867,7 @@ backup(file:string, callback: AsyncCallback<void>):void
 ```js
 let file = "BK001";
 try {
-    kvStore.backup(file, function(err) => {
+    kvStore.backup(file, (err) => {
         if (err) {
             console.error(`Failed to backup.code is ${err.code},message is ${err.message} `);
         } else {