bpo-16396: fix BPO number in changelog (GH-23951) (GH-23956)

Automerge-Triggered-By: GH:jaraco
(cherry picked from commit 7865f51)

Co-authored-by: Shantanu <12621235+hauntsaninja@users.noreply.github.com>

Author: miss-islington

Misc/NEWS.d/3.9.1rc1.rst

@@ -475,7 +475,7 @@ compression header.
 
 ..
 
-.. bpo: 16936
+.. bpo: 16396
 .. date: 2020-07-08-09-45-00
 .. nonce: z8o8Pn
 .. section: Library