Prometheus Monitoring: add missing commata at the end of the line

What does this MR do and why?

After copy-pasting the example configuration for nginx status with multiple Prometheus IPs, nginx complained about a config error due to a missing comma.

This MR updates the documentation accordingly.

Screenshots or screen recordings

This is a change in the Docs only. No UI or other changes.

How to set up and validate locally

1. copy the NGINX status example for integrating GitLab in your gitlab.rb
2. pre-MR: gitlab-ctl reconfigure will run in a config error in Nginx
3. post-MR: the config can be applied

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.