Fix link to bpo issue in Changelog (GH-17692)

diff --git a/Misc/NEWS.d/3.9.0a2.rst b/Misc/NEWS.d/3.9.0a2.rst
index 60d0ea5..50478c0 100644
--- a/Misc/NEWS.d/3.9.0a2.rst
+++ b/Misc/NEWS.d/3.9.0a2.rst
@@ -453,7 +453,7 @@
 
 ..
 
-.. bpo: 38668
+.. bpo: 38688
 .. date: 2019-11-22-10-45-03
 .. nonce: iKx23z
 .. section: Library