set credits for "Virtuelle Maschinen" to 6

this was changed meanwhile:
https://www.hpi.uni-potsdam.de/studium/lehrangebot/itse/veranstaltung/virtuelle_maschinen_und_ausfuehrungsumgebungen.html
Merge pull request #2 from 0x203/patch-1

set credits for "Virtuelle Maschinen" to 6