Cygwin: Note Ctrl-O (FLUSHO) fix to release note

Author: tyan0

winsup/cygwin/release/3.6.5

@@ -22,3 +22,6 @@ Fixes:
 
 - Make process_fd correctly handle pty and console.
  Addresses: https://cygwin.com/pipermail/cygwin/2025-May/258167.html
+
+- Fix Ctrl-O (FLUSHO) handling.
+ Addresses: https://cygwin.com/pipermail/cygwin/2025-August/258717.html