Remove ONBUILD references from Mono content.md
Remove ONBUILD references from Mono content.md

The onbuild variant was removed a long time ago, but I wasn't aware of this doc.
nats-io/docker-docs,jetty-project/docker-docs,infosiftr/docker-library-docs,docker-library/docs,jetty-project/docker-docs,Lightstreamer/docs,nats-io/docker-docs,Bonitasoft-Community/docker_docs,Lightstreamer/docs,Bonitasoft-Community/docker_docs,docker-library/docs,infosiftr/docker-library-docs