fix deprecated function

Author: yangdanny97

pandas-stubs/core/frame.pyi

@@ -2149,7 +2149,7 @@ class DataFrame(NDFrame, OpsMixin, _GetItemHack):
     def pct_change(
         self,
         periods: int = 1,
-        fill_method: None = "pad",
+        fill_method: None = None,
         freq: DateOffset | dt.timedelta | _str | None = ...,
         *,
         axis: Axis = ...,

pandas-stubs/core/series.pyi

@@ -1589,7 +1589,7 @@ class Series(IndexOpsMixin[S1], NDFrame):
     def pct_change(
         self,
         periods: int = 1,
-        fill_method: None = "pad",
+        fill_method: None = None,
         freq: DateOffset | timedelta | _str | None = ...,
         *,
         fill_value: Scalar | NAType | None = ...,