fix parameter naming

Author: Artem Tarasov

src/parquet/column/statistics.h

@@ -111,7 +111,7 @@ class PARQUET_EXPORT TypedRowGroupStatistics : public RowGroupStatistics {
 
   void PlainEncode(const T& src, std::string* dst);
   void PlainDecode(const std::string& src, T* dst);
-  void Copy(const T& src, T& dst, std::vector<uint8_t>& free_flag);
+  void Copy(const T& src, T& dst, std::vector<uint8_t>& buffer);
   std::vector<uint8_t> min_buffer_, max_buffer_;
 };