gridspec index in plot_completeness.py causes an error for 2 or fewer params

[petiay]

If one wants to plot the completeness for two or fewer parameters, when setting up the subplots this line causes an error since the returned grid spec may not have enough elements to reference the 3rd element.

For now a simple fix is to at least allow the computation for 2 parameters. Going down to one parameter will require a more thorough rearranging of the code since in this case a subplot is not even needed.

But at least this can be specified when running the code - that the fewest # of parameters required are two.