update zh-cn/application-dev/database/database-relational-guidelines.md. 修改代码格式

修改代码缩进
Signed-off-by: Ntingmac <tingmac@isoftstone.com>

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

@@ -382,17 +382,17 @@
 
     ```js
     let promiseBackup = rdbStore.backup("dbBackup.db")
-    promiseBackup.then(()=>{
+    promiseBackup.then(() => {
         console.info('Backup success.')
-    }).catch((err)=>{
+    }).catch((err) => {
         console.info('Backup failed, err: ' + err)
     })
     ```
     ```js
     let promiseRestore = rdbStore.restore("dbBackup.db")
-    promiseRestore.then(()=>{
+    promiseRestore.then(() => {
         console.info('Restore success.')
-    }).catch((err)=>{
+    }).catch((err) => {
         console.info('Restore failed, err: ' + err)
     })
     ```