Fix the beta links

[Rahul Sundaram <sundaram fedoraproject org>]

Hi

http://fedoraproject.org/en/get-fedora says beta on top and

http://fedoraproject.org/en/get-prerelease points to beta links and 
people are actively downloading them now. We need to remove the 
alpha/beta releases from the torrrent and replace the beta links with 
preview release in the website now.

Rahul