Update ubuntu/content.md with new rootfs tarball source

[toabctl]

Images newer than Xenial use a new source for the rootfs tarballs. See also https://github.com/tianon/docker-brew-ubuntu-core/commit/26b6ae75425961f283d174966c355d3eddcef9fd

[tianon]

I guess this needs to wait for https://github.com/tianon/docker-brew-ubuntu-core/pull/208 to be all the way through https://github.com/docker-library/official-images, right?

[toabctl]

I guess this needs to wait for tianon/docker-brew-ubuntu-core#208 to be all the way through https://github.com/docker-library/official-images, right?

@tianon yes. and https://github.com/tianon/docker-brew-ubuntu-core/pull/208 is ready for merging imo

[mwhudson]

It would be good to mention or reference the process by which the oci tarballs get made somehow but I don't know how to explain all this stuff in an appropriately pithy way

[yosifkit]

Closing in favor of the newer, related changes in #2228