fix a compile error

Author: JiayiFeng

doc/design/cpp_data_feeding.md

@@ -20,7 +20,7 @@ class ReaderBase {
     PADDLE_ENFORCE(!shapes_.empty());
   }
   // Read the next batch of data. (A 'batch' can be only one instance)
-  // If the next batch doesn't exist, the 'out' will be an empty std::vector.
+  // If the next batch doesn't exist, the '*out' will be an empty std::vector.
   virtual void ReadNext(std::vector<LoDTensor>* out) = 0;
 
   // Reinitialize the reader and read the file from the begin.

paddle/fluid/operators/reader/create_batch_reader_op.cc

@@ -70,7 +70,7 @@ void BatchReader::ReadNext(std::vector<framework::LoDTensor>* out) {
   for (int i = 0; i < batch_size_; ++i) {
     buffer_.push_back(std::vector<framework::LoDTensor>());
     reader_->ReadNext(&buffer_.back());
-    if (buffer.back().empty()) {
+    if (buffer_.back().empty()) {
       buffer_.pop_back();
       break;
     }