[PKT_SCHED]: fix typo on Kconfig

This is a trivial fix for a typo on Kconfig, where the Generic Random Early 
Detection algorithm is abbreviated as RED instead of GRED.

Signed-off-by: Lucas Correia Villa Real <lucasvr@gobolinux.org>
Signed-off-by: David S. Miller <davem@davemloft.net>