Programmer Passport: Elixir: Update cmd ( page 17 )

@redrapids

The example command to start using exercism is

`exercism download --exercise=hello --track=elixir`

yields the follow

mini.local >exercism download --exercise=hello --track=elixir              
Error: The exercise you specified could not be found
mini.local >

This should now be

exercism download --track=elixir --exercise=hello-world

From exercism site.