开启代理订阅功能

File: [/zh_CN/advanced/proxy-subscriptions.md](https://docs.emqx.cn/broker/latest/advanced/proxy-subscriptions.html)

原文：代理订阅功能由 emqx_mod_subscription 内置模块提供，此功能默认关闭，支持在 EMQ X Broker 运行期间动态启停，请参见 内置模块。
[内置模块] 的链接失效了。

Open zh_CN/advanced/proxy-subscriptions.md and inspect the broken 内置模块 link in the proxy subscription paragraph. Find the corresponding built-in modules documentation page, update the link, and verify that it resolves correctly in the rendered documentation.

Written by the indexing model from the issue text.