Update mongo license.md to note SSPL
Update mongo license.md to note SSPL

Closes https://github.com/docker-library/mongo/issues/324

Lightstreamer/docs,odoo/docker-docs,Bonitasoft-Community/docker_docs,infosiftr/docker-library-docs,docker-library/docs,tilosp-docker/library-docs,jetty-project/docker-docs,avoinea/docs,avoinea/docs,Lightstreamer/docs,jetty-project/docker-docs,nats-io/docker-docs,docker-library/docs,31z4/docs,docker-solr/docs,nats-io/docker-docs,crate/docker-docs,infosiftr/docker-library-docs,31z4/docs,crate/docker-docs,tilosp-docker/library-docs,docker-solr/docs,odoo/docker-docs,Bonitasoft-Community/docker_docs