iwlwifi: mvm: fix values in the table example

We erroneously had some values for NGI in the table we give as an
example in rs_fill_rates_for_column(), when they should be SGI.
Change them so that they match what we say.

Reported-by: Rémy Grünblatt <remy@grunblatt.org>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>