improve readme example
improve readme example

 - use more obvious `RAILS_ENV: :production` old one made me wonder "what does this even do, is this the ENV why is it not uppercase"
 - use more obvious `execute :rake`, misleading users into thinking all kinds of commands are prefdefined is bad, show them the general purpose tools
 - reduce example from 3->2 that's enough to show how it's done
 - remove `wait: 5` rarely used argument and users can just use builtin `sleep 5` if they want it
 - use simpler array syntax to not confuse

capistrano/sshkit