docs: more generous VM RAM/CPU size

* Updates example VM parameters to use more RAM and more CPU.

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>
global: open-data.cern.ch -> opendata.cern.ch

* Switches to the new canonical URL `opendata.cern.ch`.  (closes #62)

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>