Documentation: devlink: mlx5.rst: Fix htmldoc build warning

Fix the following build warning:

Documentation/networking/devlink/mlx5.rst:13: WARNING: Error parsing content block for the "list-table" directive:
+uniform two-level bullet list expected, but row 2 does not contain the same number of items as row 1 (2 vs 3).
...

Add the missing item in the first row.

Fixes: 0844fa5f ("net/mlx5: Let user configure io_eq_size param")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>