Update Row Grouping example to use 2D arrays

[kthorngren]

The Row Grouping example uses 1D arrays with the order() API which breaks the use of var currentOrder = table.order()[0];. This change is to use the 2D array data structure, that is consistent with the order option and clicking a column header to sort, in the event handler. This is to fix the issue found in this thread.

This contribution follows the project's existing license (MIT).

[AllanJard]

Nice one - thank you. It should be that the return from order() is consistent regardless of the input. That has actually been fixed for 2.3, but yup, I'm quite happy that the example reflects this.