fixing doctests

Author: realead

pandas/core/series.py

@@ -1873,7 +1873,7 @@ def _set_name(self, name, inplace=False) -> Series:
 >>> ser.groupby(level=0, dropna=False).sum()
 a    3
 b    3
-NaN  3
+nan  3
 dtype: int64
 
 >>> arrays = ['Falcon', 'Falcon', 'Parrot', 'Parrot']