Update deprecated nginx limit_zone to limit_conn_zone

Old link (http://wiki.nginx.org/HttpLimitZoneModule) says:

> WARNING: this article is obsoleted. Please refer to http://nginx.org/en/docs/ for the latest official documentation.

New docs for limit_zone (http://nginx.org/en/docs/http/ngx_http_limit_conn_module.html#limit_zone) say:

> This directive is made obsolete in version 1.1.8, an equivalent limit_conn_zone directive with a changed syntax should be used instead

This updates the link to go to limit_conn_zone docs instead.