fix chess960_pos doc, 0..959 not 956

Author: kraktus

chess/__init__.py

@@ -2817,7 +2817,7 @@ def set_chess960_pos(self, scharnagl: int) -> None:
 
     def chess960_pos(self, *, ignore_turn: bool = False, ignore_castling: bool = False, ignore_counters: bool = True) -> Optional[int]:
         """
-        Gets the Chess960 starting position index between 0 and 956,
+        Gets the Chess960 starting position index between 0 and 959,
         or ``None`` if the current position is not a Chess960 starting
         position.