sch_cake: revise docs for RFC 8622 LE PHB support

Commit b8392808

 ("sch_cake: add RFC 8622 LE PHB support to CAKE
diffserv handling") added the LE mark to the Bulk tin. Update the
comments to reflect the change.

Signed-off-by: Tyson Moore <tyson@tyson.me>
Acked-by: Toke Høiland-Jørgensen <toke@toke.dk>
Signed-off-by: David S. Miller <davem@davemloft.net>