fix: use default value of 10

sync default value with docappender default

internal/beater/beater.go

@@ -700,6 +700,7 @@ func (s *Runner) newFinalBatchProcessor(
 	}
 	esConfig.FlushInterval = time.Second
 	esConfig.Config = elasticsearch.DefaultConfig()
+	esConfig.MaxIdleConnsPerHost = 10
 	if err := s.elasticsearchOutputConfig.Unpack(&esConfig); err != nil {
 		return nil, nil, err
 	}