修改preferences示例代码错误

Signed-off-by: Nli_juntao <lijuntao9@huawei.com>

zh-cn/application-dev/reference/apis/js-apis-data-preferences.md

@@ -603,14 +603,16 @@ on(type: 'change', callback: Callback<{ key : string }>): void
 
 **示例：**
 ```ts
-var observer = function (key) {
+data_preferences.getPreferences(this.context, 'mystore', function (err, preferences) {
+    if (err) {
+        console.info("Get preferences failed.")
+        return;
+    }
+    var observer = function (key) {
         console.info("The key of " + key + " changed.")
-}
-
-...
-
-preferences.on('change', observer)
-preferences.put('startup', 'auto', function (err) {
+    }
+    preferences.on('change', observer)
+    preferences.put('startup', 'auto', function (err) {
         if (err) {
             console.info("Put the value of startup failed, err: " + err)
             return
@@ -624,6 +626,7 @@ preferences.put('startup', 'auto', function (err) {
             }
             console.info("Flushed to file successfully.") // observer will be called.
         })
+    })
 })
 ```
 
@@ -644,14 +647,16 @@ off(type: 'change', callback?: Callback<{ key : string }>): void
 
 **示例：**
 ```ts
-var observer = function (key) {
+data_preferences.getPreferences(this.context, 'mystore', function (err, preferences) {
+    if (err) {
+        console.info("Get preferences failed.")
+        return;
+    }
+    var observer = function (key) {
         console.info("The key of " + key + " changed.")
-}
-
-...
-
-preferences.on('change', observer)
-preferences.put('startup', 'auto', function (err) {
+    }
+    preferences.on('change', observer)
+    preferences.put('startup', 'auto', function (err) {
         if (err) {
             console.info("Put the value of startup failed, err: " + err)
             return
@@ -664,6 +669,7 @@ preferences.put('startup', 'auto', function (err) {
                 return
             }
             console.info("Flushed to file successfully.") // observer will be called.
+        })
         preferences.off('change', observer)
     })
 })