change fill_value param to scalar

Author: KevsterAmp

pandas/core/frame.py

@@ -9854,7 +9854,7 @@ def unstack(
         ----------
         level : int, str, or list of these, default -1 (last level)
             Level(s) of index to unstack, can pass level name.
-        fill_value : int, str or dict
+        fill_value : scalar
             Replace NaN with this value if the unstack produces missing values.
         sort : bool, default True
             Sort the level(s) in the resulting MultiIndex columns.