message: Corrected the docs for shift
diff: @@ -83,7 +83,7 @@ self_shift(size_type periods, shift_policy sp);
  <PRE><B>
  template&lt;typename ... Ts&gt;
  StdDataFrame&lt;I&gt;
  -shift(size_type periods, shift_policy sp); </font>
  +shift(size_type periods, shift_policy sp) const; </font>
  </B></PRE>
  </td>
  <td>
  