update docs

Signed-off-by: NAnnie_wang <annie.wangli@huawei.com>

en/application-dev/database/database-preference-guidelines.md

@@ -114,21 +114,19 @@ You can use the following APIs to delete a **Preferences** instance or data file
    ```ts
    // Obtain the context.
    import UIAbility from '@ohos.app.ability.UIAbility';
-   let context = null;
    let preferences = null;
    export default class EntryAbility extends UIAbility {
-       onWindowStageCreate(windowStage){
-           context = this.context;
-       }
-   }
-   
-   let promise = data_preferences.getPreferences(context, 'mystore');
-   
+       onWindowStageCreate(windowStage) {
+           let promise = data_preferences.getPreferences(this.context, 'mystore');
             promise.then((pref) => {
                 preferences = pref;
             }).catch((err) => {
                 console.info("Failed to get the preferences.");
             })
+       }
+   }
+   
+   
    ```
 
 3. Write data.

en/application-dev/reference/apis/js-apis-data-preferences.md

@@ -22,8 +22,8 @@ import data_preferences from '@ohos.data.preferences';
 
 | Name            | Type| Readable| Writable| Description                                   |
 | ---------------- | -------- | ---- | ---- | --------------------------------------- |
-| MAX_KEY_LENGTH   | number   | Yes  | No  | Maximum length of a key. The key must be less than 80 bytes.    |
-| MAX_VALUE_LENGTH | number   | Yes  | No  | Maximum length of a value. The value must be less than 8192 bytes.|
+| MAX_KEY_LENGTH   | number   | Yes  | No  | Maximum length of a key. The maximum key length is 80 bytes.    |
+| MAX_VALUE_LENGTH | number   | Yes  | No  | Maximum length of a value. The maximum value length is 8192 bytes.|
 
 
 ## data_preferences.getPreferences