Fix off by one error on loop indices in python version of example 21.  Results now agree with the C version.
Fix off by one error on loop indices in python version of example 21. 
Results now agree with the C version.


svn path=/trunk/; revision=8650

FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot