libtmux(docs[session]): fix kill_window docstring type to match signature

why: Docstring said "str, optional" but the actual signature accepts
str | int | None.
what:
- Change target_window type in docstring to "str | int, optional"

Author: tony

src/libtmux/session.py

@@ -606,7 +606,7 @@ def kill_window(self, target_window: str | int | None = None) -> None:
 
         Parameters
         ----------
-        target_window : str, optional
+        target_window : str | int, optional
             Window to kill.
 
         Raises