Update Readme for Windows
The readme has a line linking to a recommended Octave version (https://github.com/DynareTeam/dynare/blob/48b5839414b96c69c0ea0a0da4b129b0195d7dfe/windows/README.txt#L64), which should be 4.2.1, but http://www.dynare.org/download/octave/windows only has downloads for 3.8.1