message: update example to use get_resume_token
diff: @@ -449,10 +449,10 @@ TEST_CASE("Documentation Examples", "[min36]") {
  SECTION("Example 3") {
  // Start Changestream Example 3
  - stdx::optional<bsoncxx::document::view_or_value> resume_token;
  + stdx::optional<bsoncxx::document::view> resume_token;
  change_stream stream = inventory.watch();
  for (auto& event : stream) {
  - resume_token = bsoncxx::document::view_or_value{event["_id"].get_document()};
  + resume_token = stream.get_resume_token();
  }
  if (resume_token) {
  