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.