hw/display/cirrus_vga: Update the documentation URL

The documentation URL is not working, but is backed up by the
Wayback Machine on the Internet Archive.
Replace the outdated link by a captured one.
Add another link to the VGADOC4b.ZIP archive content.

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20190504121650.12651-1-philmd@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

hw/display/cirrus_vga.c

@@ -23,8 +23,13 @@
  * THE SOFTWARE.
  */
 /*
- * Reference: Finn Thogersons' VGADOC4b
- *   available at http://home.worldonline.dk/~finth/
+ * Reference: Finn Thogersons' VGADOC4b:
+ *
+ *  http://web.archive.org/web/20021019054927/http://home.worldonline.dk/finth/
+ *
+ * VGADOC4b.ZIP content available at:
+ *
+ *  https://pdos.csail.mit.edu/6.828/2005/readings/hardware/vgadoc
  */
 #include "qemu/osdep.h"
 #include "qemu/units.h"