Issue #24952: Clarify default argument of stack_size() in threading, thread

Patch from Mattip.
diff --git a/Misc/ACKS b/Misc/ACKS
index d732bf0..ba3e3fe 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -875,6 +875,7 @@
 Simon Mathieu
 Laura Matson
 Graham Matthews
+mattip
 Martin Matusiak
 Dieter Maurer
 Daniel May