Fix iex dialyzer warnings

Author: fishcakez

lib/iex/lib/iex/server.ex

@@ -77,7 +77,7 @@ defmodule IEx.Server do
   we spawn a new server for it without waiting for its
   conclusion.
   """
-  @spec start(list, mfa) :: :ok
+  @spec start(list, {module, atom, [any]}) :: :ok
   def start(opts, {m, f, a}) do
     {pid, ref} = spawn_monitor(m, f, a)
     start_loop(opts, pid, ref)