net/mlx5: docs: correct section reference in table of contents

Commit 142d93d1

 ("net/mlx5: Add devlink subfunction port
documentation") refers to a section 'mlx5 port function' in the table of
contents, but includes a section 'mlx5 function attributes' instead.

Hence, make htmldocs warns:

  mlx5.rst:16: WARNING: Unknown target name: "mlx5 port function".

Correct the section reference in table of contents to the actual name of
section in the documentation.

Also, tune another section underline while visiting this document.

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