fixed 3accdca9 from https://gitee.com/wuliubao/docs/pulls/2657

IssueNo:#I4ZZI2
Description:添加serviceExtAbility链接
Sig: SIG_ApplicationFramework
Feature or Bugfix: Feature
Binary Source: No
Signed-off-by: wuliubao <ahjxliubao@163.com>

zh-cn/application-dev/ability/stage-serviceextension.md

@@ -80,7 +80,7 @@ ExtensionAbility，是Stage模型中新增的扩展组件的基类，一般用
 
 针对ServiceExtensionAbility开发，有以下示例工程可供参考：
 
-- [ServiceExtensionAbility]()
+- [ServiceExtensionAbility](https://gitee.com/openharmony/app_samples/tree/master/ability/eTSServiceExtAbility)
 
 本示例ServiceExtensionAbility中：