net/mlx5: fix config name in Kconfig parameter documentation

Commit a12ba192 ("net/mlx5: Update Kconfig parameter documentation")
adds documentation on Kconfig options for the mlx5 driver. It refers to the
config MLX5_EN_MACSEC for MACSec offloading, but the config is actually
called MLX5_MACSEC.

Fix the reference to the right config name in the documentation.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>