[FIXED HUDSON-7988] 404 Not Found when "Downgrade to XX" button was clicked.

git-svn-id: https://hudson.dev.java.net/svn/hudson/trunk/hudson/main@36598 71c3de6d-444a-0410-be80-ed276b4c234a

core/src/main/java/hudson/model/UpdateSite.java

@@ -622,7 +622,7 @@ public class UpdateSite {
          */
         public void doDowngrade(StaplerResponse rsp) throws IOException {
             deployBackup();
-            rsp.sendRedirect("../updateCenter/");
+            rsp.sendRedirect2("../..");
         }
     }