[plt-scheme] Web page change

FYI --

The pre-release content moved to a more proper place, it is now at
http://pre.plt-scheme.org, with the installers at the usual
subdirectory.

The old location (download.plt-scheme.org/scheme) will redirect you to
the new one.  Also links to deeper in the old location (like links to
docs and sources) will continue to function for a few months.

-- 
          ((lambda (x) (x x)) (lambda (x) (x x)))          Eli Barzilay:
                  http://www.barzilay.org/                 Maze is Life!