update zh-cn/application-dev/database/database-relational-guidelines.md.

Signed-off-by: N@ge-yafang <geyafang@huawei.com>

zh-cn/application-dev/database/database-relational-guidelines.md

@@ -263,7 +263,7 @@
     let predicate = new data_rdb.RdbPredicates('test')
     predicate.inDevices(['12345678abcde'])
     let promise = rdbStore.sync(rdb.SyncMode.SYNC_MODE_PUSH, predicate)
-    promise.then(result) {
+    promise.then((result) => {
         console.log('sync done.')
         for (let i = 0; i < result.length; i++) {
             console.log('device=' + result[i][0] + ' status=' + result[i][1])