New nginx path in installation instructions

[axzinker]

Just a small correction for the installation instructions for configuring nginx:

https://bigbluebutton-exporter.greenstatic.dev/installation/all_in_one_monitoring_stack/#5-configure-nginx

The new file should be in

/usr/share/bigbluebutton/nginx/monitoring.nginx

but not in

/etc/bigbluebutton/nginx/monitoring.nginx

(otherwise Greenlight etc. won't work any more)