nixos.ipxe: fix version mistake

Selecting the NixOS 19.09 entry was booting 19.03 because of a mistake

src/nixos.ipxe

@@ -6,7 +6,7 @@
 set os Nixos Linux
 menu ${os}
 item --gap Official Releases
-item 19.03 ${space} Nixos 19.09
+item 19.09 ${space} Nixos 19.09
 item 19.03 ${space} Nixos 19.03
 item unstable ${space} Nixos unstable
 choose version || goto nixos_exit