ALICE OpenData instructions still reference "CernVM-Online"

[vvolkl]

As was brought up by a post on the CernVM-FS forum, the page

https://opendata.cern.ch/docs/alice-virtual-machine#vbox

instructs users to get contexts from cernvm-online. However, this service has been deprecated since a long time and was finally removed (See this post for more information: https://cernvm-forum.cern.ch/t/decommissioning-of-the-cernvm-online-web-site/264). The page should be updated to provide a context that can be used to create ALICE opendata vms. If we (the CVMFS dev team) can help, do let us know!