index.html: Expand pie-chart slides to 600px high

index.html: Expand pie-chart slides to 600px high

Which gives room to grow the pie charts to 250px.  The 600px height
matches css/reveal.css:

  .reveal .slides {
    ...
    	        perspective: 600px;
    ...
    }

and similar.  After the change, the pie-chart slides are the same
height as the other slides when viewed in the slide overview (press
ESC).