Add regression issue number to Misc/NEWS entry.

Misc/NEWS

@@ -10,10 +10,10 @@ What's New in Python 2.7.9?
 Core and Builtins
 -----------------
 
-- Backout issue #1856 patch (avoid crashes and lockups when daemon threads
-  run while the interpreter is shutting down; instead, these threads are
-  now killed when they try to take the GIL), as it seems to break some
-  existing code.
+- Issue #21963: backout issue #1856 patch (avoid crashes and lockups when
+  daemon threads run while the interpreter is shutting down; instead, these
+  threads are now killed when they try to take the GIL), as it seems to
+  break some existing code.
 
 - Issue #22604: Fix assertion error in debug mode when dividing a complex
   number by (nan+0j).