docs: limit-conn plugin in stream proxy

### Improved or typo fixed.

Currently, the limit-conn is supported in the stream proxy, but the documentation about limit-conn doesn't give a good example to show how to use it in stream proxy.

Documentation link: http://apisix.apache.org/docs/apisix/plugins/limit-conn.

### Solution

We may add an example for how to use limit-conn in the stream proxy.

Open the linked limit-conn documentation page and review its current examples alongside the stream proxy support described in the issue. Add a clear stream-proxy usage example and verify that the page explains how to configure limit-conn for that use case.

Written by the indexing model from the issue text.