[doc] Issue in "Image configuration" - Kiwi mis-match

[peteingithub]

Image configuration:

https://documentation.suse.com/suse-edge/3.3/html/edge/quickstart-metal3.html#id-image-configuration

"The base image when downloaded is xz compressed, which must be uncompressed with unxz and copied/moved under the base-images folder."

Changed with Kiwi.