Surface_Water_Mass_Transformation.ipynb should use pme_net not pme_river

[adele-morrison]

@kialstewart and @aekiss can confirm, but based on our discussion today and Kial's analysis of the ACCESS-OM2 freshwater diagnostics, I think we should be using pme_net instead of pme_river for the total freshwater forcing in Surface_Water_Mass_Transformation.ipynb. The difference between these two terms is the correction added by MOM5 to try to balance the net mass in the combined sea ice plus ocean system due to imbalances arising from precip / evap differences.