* lib/webrick/httprequest.rb: [DOC] Change document link

http://Web.Golux.Com/coar/cgi/ doesn't seem to work now.
It is currently redirected to [Apache Software webring](http://www.webring.org/hub/apachesupport?w=1440;rh=http%3A%2F%2Fcgi-spec.golux.com%2F;rd=1), which is not directly referring to CGI.

And RFC 3875 does not contain this link. So I propose to change the link to RFC's.