Update packages/bigframes/bigframes/dataframe.py

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

Author: TrevorBergeron

packages/bigframes/bigframes/dataframe.py

@@ -2418,7 +2418,7 @@ def sort_index(
         *,
         ascending: bool = ...,
         inplace: Literal[False] = ...,
-        kind: str = ...,
+        kind: str | None = ...,
         na_position: Literal["first", "last"] = ...,
     ) -> DataFrame: ...