fix: Update data/dimensionless_mappings.yaml

Better example in the example. SST isn't meaningful for this use case, we instead will test with salinity of the ocean (variable `so`)

Co-authored-by: Miguel <[email protected]>

data/dimensionless_mappings.yaml

@@ -1,5 +1,5 @@
 # In general:
 # model_variable_name:
 #   cmor_unit_string: pint_friendly_SI_units
-sst:
+so:
   "0.001": g/kg