Correct 'Ubuntu' to 'minimal busybox'
Correct 'Ubuntu' to 'minimal busybox'

The busybox container in the examples is not an actual Ubuntu installation; this change updates the text to reflect this.
bq-xiao/docs,coreos/docs,marineam/coreos-docs,mjg59/docs,hkjn/docs-1,atxwebs/docs,michelleN/docs,endocode/coreos-docs,skynet/docs,robszumski/docs,joshix/docs,mischief/coreos-docs,iankronquist/docs,ionave/docs,wli6/docs,jonboulle/docs,ksmaheshkumar/docs-2,jdloft/docs