template: Fix link to CMS-specfic CernVM

Addresses #185
OK, I'll merge "as is" for today's deadline, but has anyone had a chance to discuss with the producers of the VM image about whether we couldn't have the files "properly" numbered according to the semantic versioning schema? See https://github.com/cernopendata/opendata.cern.ch/issues/185#issuecomment-55721523