Trivial: fix commandline help message

Fix obvious typo in printed help for qemu-nbd.

Signed-off-by: Andrey Korolyov <andrey@xdel.ru>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

qemu-nbd.c

@@ -102,7 +102,7 @@ static void usage(const char *name)
 "      --aio=MODE            set AIO mode (native or threads)\n"
 #endif
 "      --discard=MODE        set discard mode (ignore, unmap)\n"
-"      --detect-zeroes=MODE  set detect-zeroes mode (off, on, discard)\n"
+"      --detect-zeroes=MODE  set detect-zeroes mode (off, on, unmap)\n"
 "\n"
 "Report bugs to <qemu-devel@nongnu.org>\n"
     , name, NBD_DEFAULT_PORT, "DEVICE");