change roundup-tracker.org to www.roundup-tracker.org.

The latter is the perferred entry point.

Author: rouilj

doc/announcement.txt

@@ -96,7 +96,7 @@ then open the url printed by the demo app.
 Release info and download page:
      https://pypi.org/project/roundup
 Source and documentation is available at the website:
-     https://roundup-tracker.org/
+     https://www.roundup-tracker.org/
 Mailing lists - the place to ask questions:
      https://sourceforge.net/p/roundup/mailman/