Fix label references

[bobidle]

trafficstars

I tried to build the docs via the Docker image from Docker Hub https://hub.docker.com/r/rsyslog/rsyslog_doc_gen

And it always failed with

Warning, treated as error:
/rsyslog-doc/source/configuration/modules/omhttp.rst:634:undefined label: '_statistic_counter_label'

I tried to correct the label references with this commit/pull request.