[glibmm] NEWS: Add a missing issue number

[Kjell Ahlstedt <kjellahl src gnome org>]

commit de6b217f5f4869c6659654a6f3a4f39835405de9
Author: Kjell Ahlstedt <kjellahlstedt gmail com>
Date:   Mon Oct 7 11:16:36 2019 +0200

    NEWS: Add a missing issue number

 NEWS | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/NEWS b/NEWS
index 18fb92bd..96a2bd6c 100644
--- a/NEWS
+++ b/NEWS
@@ -32,7 +32,7 @@ Gio:
 * Drive, MountOperation, ThemedIcon, TlsDatabase, VolumeMonitor:
   Fix ownership of some lists and arrays (Fixes memory leaks
   and dangling pointers)
-  (Kjell Ahlstedt)
+  (Kjell Ahlstedt) Issue #50 (Gary Wang)
 * Add SocketControlMessage::add_deserialize_func()
   (Kjell Ahlstedt) Issue #52 (Ankur deep jaiswal)