Clarify elixir from-scratch example

[ephetic]

In https://github.com/nanobox-io/nanobox-guides/blob/master/articles/elixir/phoenix/from-scratch.md:

• it's unclear why you cd /tmp prior to installing phx
• the formatting on the following should indicate the parenthetical is not part of the command (abundance of caution)

mix phx.new app (Do not fetch dependencies at this moment)