Apply cwhanse suggestion

Co-authored-by: Cliff Hansen <cwhanse@sandia.gov>

Author: echedey-ls

pvlib/irradiance.py

@@ -575,7 +575,7 @@ def get_ground_diffuse(surface_tilt, ghi, albedo=.25, surface_type=None):
         overridden if surface_type is supplied.
 
     surface_type : string, optional
-        If not None, overrides albedo. String can be one of 'urban',
+        If supplied, overrides ``albedo``. String can be one of 'urban',
         'grass', 'fresh grass', 'snow', 'fresh snow', 'asphalt', 'concrete',
         'aluminum', 'copper', 'fresh steel', 'dirty steel', 'sea'.