[haproxy] fix example of haproxy.cfg in README.md
[haproxy] fix example of haproxy.cfg in README.md

hiroakis/mackerel-agent-plugins,hiroakis/mackerel-agent-plugins,mackerelio/mackerel-agent-plugins,hiroakis/mackerel-agent-plugins,hiroakis/mackerel-agent-plugins,mackerelio/mackerel-agent-plugins,mackerelio/mackerel-agent-plugins,mackerelio/mackerel-agent-plugins