updated mix.exs in intro to mix

Author: southkeys

getting-started/mix-otp/introduction-to-mix.markdown

@@ -90,8 +90,8 @@ defmodule KV.MixProject do
  [
 app::kv,
 version:"0.1.0",
-elixir:"~> 1.9",
-start_permanent:Mix.env==:prod,
+elixir:"~> 1.11",
+start_permanent:Mix.env()==:prod,
 deps:deps()
  ]
 end