Change nginx status section in readme

[momu]

The nginx now (at least in GitLab 11.4) comes with an own status page. See https://docs.gitlab.com/omnibus/settings/nginx.html#enablingdisabling-nginx_status

The default values are ok, so only

[nginx*]
env.url http://localhost:8060/nginx_status

has to be appended to /etc/munin/plugin-conf.d/munin-node