mlxsw: reg: Fix a typo in a group heading

There is no such thing as "traffic group". The group that this is a heading
of is "per traffic class counters". Fix the heading.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>