修改preference文档

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

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

@@ -94,9 +94,16 @@
    ```js
    // 获取context
    import featureAbility from '@ohos.ability.featureAbility'
-   let context = featureAbility.getContext()
+   let context = featureAbility.getContext();
    
+   let preferences = null;
    let promise = data_preferences.getPreferences(context, 'mystore');
+   
+   promise.then((pref) => {
+       prefrences = pref;
+   }).catch((err) => {
+       console.info("Failed to get preferences.");
+   })
    ```
 
    Stage模型示例：
@@ -104,14 +111,21 @@
    ```ts
    // 获取context
    import Ability from '@ohos.application.Ability'
-   let context = null
+   let context = null;
+   let preferences = null;
    export default class MainAbility extends Ability {
        onWindowStageCreate(windowStage){
-           context = this.context
+           context = this.context;
        }
    }
    
    let promise = data_preferences.getPreferences(context, 'mystore');
+   
+   promise.then((pref) => {
+       prefrences = pref;
+   }).catch((err) => {
+       console.info("Failed to get preferences.");
+   })
    ```
 
 3. 存入数据。
@@ -119,16 +133,12 @@
    使用put方法保存数据到缓存的实例中。
 
    ```js
-   promise.then((preferences) => {
    let putPromise = preferences.put('startup', 'auto');
    putPromise.then(() => {
        console.info("Succeeded in putting the value of 'startup'.");
    }).catch((err) => {
        console.info("Failed to put the value of 'startup'. Cause: " + err);
    })
-   }).catch((err) => {
-       console.info("Failed to get preferences.");
-   })
    ```
    
 4. 读取数据。
@@ -136,16 +146,12 @@
    使用get方法读取数据。
 
    ```js
-   promise.then((preferences) => {
    let getPromise = preferences.get('startup', 'default');
    getPromise.then((value) => {
        console.info("The value of 'startup' is " + value);
    }).catch((err) => {
        console.info("Failed to get the value of 'startup'. Cause: " + err);
    })
-   }).catch((err) => {
-     console.info("Failed to get preferences.")
-   });
    ```
    
 5. 数据持久化。
@@ -165,7 +171,7 @@
        console.info("The key" + key + " changed.");
    }
    preferences.on('change', observer);
-   // 数据产生变更，由'auto'变为'manual'。
+   // 数据产生变更，由'auto'变为'manual'
    preferences.put('startup', 'manual', function (err) {
        if (err) {
            console.info("Failed to put the value of 'startup'. Cause: " + err);