Fixing Pie Chart example to display pie charts

I have noticed that this example is currently throwing the following error when opening the produced Excel Spreadsheet -  "We found a problem with some content in <filename>.xlsx. Do you want us to try to recover as much as we can? If you trust the source of this workbook click Yes". 

In order to fix this I have set the removed the PlotGrouping setting (currently PHPExcel_Chart_DataSeries::GROUPING_STANDARD) to NULL and it appears to work.
Merge pull request #435 from masanaikeshima/patch-1

Fixing Pie Chart example to display pie charts