Documentation: net/mlx5: Wrap vnic reporter devlink commands in code blocks

Sphinx reports htmldocs warnings:

Documentation/networking/device_drivers/ethernet/mellanox/mlx5/devlink.rst:287: WARNING: Unexpected indentation.
Documentation/networking/device_drivers/ethernet/mellanox/mlx5/devlink.rst:288: WARNING: Block quote ends without a blank line; unexpected unindent.
Documentation/networking/device_drivers/ethernet/mellanox/mlx5/devlink.rst:290: WARNING: Unexpected indentation.

Fix above warnings by wrapping diagnostic devlink commands in "vnic
reporter" section in code blocks to be consistent with other devlink
command snippets.

Fixes: b0bc615d ("net/mlx5: Add vnic devlink health reporter to PFs/VFs")
Fixes: cf14af14

 ("net/mlx5e: Add vnic devlink health reporter to representors")
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>