Update default fillna method in the docstring

Author: herrfz

pandas/core/frame.py

@@ -3233,7 +3233,7 @@ def fillna(self, value=None, method=None, axis=0, inplace=False,
 
         Parameters
         ----------
-        method : {'backfill', 'bfill', 'pad', 'ffill', None}, default 'pad'
+        method : {'backfill', 'bfill', 'pad', 'ffill', None}, default None
             Method to use for filling holes in reindexed Series
             pad / ffill: propagate last valid observation forward to next valid
             backfill / bfill: use NEXT valid observation to fill gap