Add @impl to callbacks in examples (#1313)

Author: vsyrovat

getting-started/mix-otp/genserver.markdown

@@ -305,6 +305,7 @@ def handle_info({:DOWN, ref, :process, _pid, _reason},{names, refs}) do
 {:noreply,{names, refs}}
 end
 
+@impltrue
 defhandle_info(_msg, state) do
 {:noreply, state}
 end

getting-started/mix-otp/supervisor-and-application.markdown

@@ -41,6 +41,7 @@ defmodule KV.Supervisor do
 Supervisor.start_link(__MODULE__, :ok, opts)
 end
 
+@impltrue
 definit(:ok) do
  children = [
 KV.Registry