qemu-nbd: Fix wrong description in qemu-nbd.texi

-c option needs argument <dev> but it's missing now.
This patch fixes it.
Signed-off-by: NRyota Ozaki <ozaki.ryota@gmail.com>
Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>

qemu-nbd.texi

@@ -30,8 +30,8 @@ Export Qemu disk image using NBD protocol.
   use snapshot file
 @item -n, --nocache
   disable host cache
-@item -c, --connect
-  connect FILE to NBD device DEV
+@item -c, --connect=@var{dev}
+  connect @var{filename} to NBD device @var{dev}
 @item -d, --disconnect
   disconnect the specified device
 @item -e, --shared=@var{num}