recover Isabelle symlink for public distribution, notably website;

--- a/Admin/makedist	Mon Mar 14 16:59:37 2011 +0100
+++ b/Admin/makedist	Tue Mar 15 13:03:54 2011 +0100
@@ -206,6 +206,8 @@
 mkdir "$DISTNAME/doc"
 mv "${DISTNAME}-old/doc/"*.pdf "${DISTNAME}-old/doc/Contents" "$DISTNAME/doc"
 
+rm -f Isabelle && ln -sf "$DISTNAME" Isabelle
+
 chgrp -R isabelle "$DISTNAME"
 
 rm -rf "${DISTNAME}-old"