Fix underscored variable warning

Author: parroty

lib/exvcr/actor.ex

@@ -55,7 +55,7 @@ defmodule ExVCR.Actor do
 
     use ExActor.GenServer, export: __MODULE__
 
-    defstart(start_link(_arg), do: default_state() |> initial_state())
+    defstart(start_link(_), do: default_state() |> initial_state())
 
     defcast(set(x), do: new_state(x))
     defcall(get, state: state, do: reply(state))