fix(using-coreos): fix sudo command
fix(using-coreos): fix sudo command

the - from 'sudo su -' was missing and @jeremyvisser pointed out that
sudo -i is equivalent. Use that instead. Fixes #13

marineam/coreos-docs,jdloft/docs,jonboulle/docs,skynet/docs,michelleN/docs,endocode/coreos-docs,joshix/docs,hkjn/docs-1,atxwebs/docs,mischief/coreos-docs,mjg59/docs,coreos/docs,iankronquist/docs,robszumski/docs,ksmaheshkumar/docs-2,bq-xiao/docs,ionave/docs,wli6/docs