chore(authors): update authors list

Author: BeibeiX0

Mathlib/Combinatorics/Enumerative/Stirling.lean

@@ -1,7 +1,7 @@
 /-
 Copyright (c) 2025 Beibei Xiong. All rights reserved.
 Released under Apache 2.0 license as described in the file LICENSE.
-Authors: Beibei Xiong, Yu Shao, Weijie Jiang
+Authors: Beibei Xiong, Yu Shao, Weijie Jiang, Zhengfeng Yang
 -/
 import Mathlib.Tactic.Ring
 import Mathlib.Data.Nat.Factorial.Basic