message: Update README.md
  Fixed outdated issue in the doc
diff: @@ -20,7 +20,7 @@ Instances of DataFrame are not multithreaded safe. In other words, a single inst
  <B>Example Code</B>
  ```CPP
  // Define a DataFrame with unsigned long index type that uses std::vector
  -typedef DataFrame<unsigned long, std::vector> MyDataFrame;
  +typedef StdDataFrame<unsigned long> MyDataFrame;
  MyDataFrame df;
  std::vector<int> intvec = { 1, 2, 3, 4, 5 };
  