[patch][doc] Fix EXAMPLE in kqueue(2)

The error control was not properly implemented. "changelist" is const, hence
event.flags is never changed by the syscall.

PR: 196844
Reported by: eugen@
Reviewed by: PauAmma <pauamma@gundo.com>
Approved by: eugen@
Fixes: 8c231786f01b9f8614e2fe5b47196db1caa7a772