fix write_to typespec

lib/elixlsx.ex

@@ -43,7 +43,7 @@ defmodule Elixlsx do
   @doc ~S"""
   Write a Workbook object to the given filename.
   """
-  @spec write_to(Elixlsx.Workbook.t(), String.t()) :: {:ok, String.t()} | {:error, any()}
+  @spec write_to(Elixlsx.Workbook.t(), String.t()) :: {:ok, charlist} | {:error, any()}
   def write_to(workbook, filename) do
     wci = Elixlsx.Compiler.make_workbook_comp_info(workbook)
     :zip.create(to_charlist(filename), Elixlsx.Writer.create_files(workbook, wci))