Update ko/basics/functions.md (#2975)

It's same as a6d86ef and seems this code is not working on newer elixir

lessons/ko/basics/functions.md

@@ -283,7 +283,7 @@ end
 
 ```elixir
 defmodule Greeter do
-  def hello(name), do: phrase <> name
+  def hello(name), do: phrase() <> name
   defp phrase, do: "Hello, "
 end