message: Update the error message for invalid buffer_size
diff: @@ -386,7 +386,10 @@ void PrefetchDatasetOp::MakeDataset(OpKernelContext* ctx, DatasetBase* input,
  ParseScalarArgument<int64>(ctx, "buffer_size", &buffer_size));
  OP_REQUIRES(ctx,
  buffer_size >= 0 || buffer_size == PrefetchAutotuner::kAutoTune,
  - errors::InvalidArgument("buffer_size must be >= 0"));
  + errors::InvalidArgument("buffer_size must be >= 0 or set "
  + "buffer_size to be ",
  + PrefetchAutotuner::kAutoTune,
  + " for auto-tuning"));
  if (buffer_size == PrefetchAutotuner::kAutoTune) {
  metrics::RecordTFDataAutotune(kDatasetName);
  