rich media: parser: add copyright header

lib/pleroma/web/rich_media/parser.ex

+# Pleroma: A lightweight social networking server
+# Copyright © 2017-2019 Pleroma Authors <https://pleroma.social/>
+# SPDX-License-Identifier: AGPL-3.0-only
+
 defmodule Pleroma.Web.RichMedia.Parser do
   @parsers [
     Pleroma.Web.RichMedia.Parsers.OGP,