Betzy: Corrected GPU node config

The queue system configuration of the GPU nodes on Betzy had an error: The number of CPUs were set to 128 instead of 64. Most jobs would probably not be affected by this, but it is possible that some jobs got sub-optimal cpu pinnings.

This has now been fixed, and the documentation updated. There is nothing users have to do with their job scripts (except if they asked for more than 64 cpus per node).