doc: fix typo (#10409)

Fix minor typo.

Mathlib/SetTheory/Game/Impartial.lean

@@ -13,7 +13,7 @@ import Mathlib.Tactic.NthRewrite
 
 We will define an impartial game, one in which left and right can make exactly the same moves.
 Our definition differs slightly by saying that the game is always equivalent to its negative,
-no matter what moves are played. This allows for games such as poker-nim to be classifed as
+no matter what moves are played. This allows for games such as poker-nim to be classified as
 impartial.
 -/