Mix and OTP guide: mix test does not run in "Doctests, patterns and with"

[FumioNonaka]

https://github.com/elixir-lang/elixir-lang.github.com/edit/master/getting-started/mix-otp/docs-tests-and-with.markdown#L73
The following error is shown:

** (Mix) Could not start application kv_server: exited in: KVServer.Application.start(:normal, [])
 ** (EXIT) an exception was raised:
 ** (RuntimeError) missing $PORT environment variable

It is caused by the code below in the KVServer.Application.start/2.

port = String.to_integer(System.get_env("PORT") || raise "missing $PORT environment variable")

I changed the line into:

port = String.to_integer(System.get_env("PORT") || "4040")

[fidenz-chim]

Added this modification to the documentation.