Update nginx.md, workaround for issue 2854, SSL certificate updates without downtime
requested to merge BobFrankly/omnibus-gitlab:i2854d-DocumentationOfWorkaroundForSSL_KeyUpdates into master
Documenting a workaround to issue #2854 (closed) .
Branch is purposefully misnamed to avoid closing the issue, as this is only documentation to help someone else avoid the time I lost discovering this issue.