man: tc-pedit: Drop the claim that pedit ex is only for IPv4

This sentence predates addition of extended pedit for IPv6 packets.

Reported-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David Ahern <dsahern@gmail.com>