Fix link of CODE_OF_CONDUCT.md in pull request template (#2735)

.github/pull_request_template.md

@@ -17,4 +17,4 @@ Side effects:
 ### Check List:
 - Please make sure your changes are compilable(请确保你的更改可以通过编译).
 - When providing us with a new feature, it is best to add related tests(如果你向我们增加一个新的功能, 请添加相关测试).
-- Please follow [Contributor Covenant Code of Conduct](../../master/CODE_OF_CONDUCT.md).(请遵循贡献者准则).
+- Please follow [Contributor Covenant Code of Conduct](https://github.com/apache/brpc/blob/master/CODE_OF_CONDUCT.md).(请遵循贡献者准则).