Update nerdcore.php

updates feed url to nerdcore.de/feed/ cause nerdcore.de/?feed=rss2 redirects to it and not to feedburner anymore (although feedburner url is still linked on top of the page)
this fixes the "WARNING! Your Rss-Extender rules returned an empty string for link: //feedproxy.google.com/~r/NerdcoreRSS2/~3/jKYOMATP8k4/" produced by feedburner