Fix stretch image format in rpios-stretch configuration file

Author: joolswills

scriptmodules/admin/image/dists/rpios-stretch.ini

@@ -1,7 +1,7 @@
 name="stretch"
 version="9"
 url="https://downloads.raspberrypi.org/raspbian_lite/images/raspbian_lite-2019-04-09/2019-04-08-raspbian-stretch-lite.zip"
-format="xz"
+format="zip"
 platforms="rpi1 rpi2"
 file_rpi1="rpi1_zero"
 title_rpi1="RPI 1/Zero"