Fixed Rocket.pde of EvolveFlowField: ArrayIndexOutOfBoundsException

When I ran this example and some rockets came to the bottom right corner of the window, it threw an `ArrayIndexOutOfBoundsException` at that line.

Actually
`y*(width/gridscale)`should be the same as
`y*width/gridscale`.

But as `y` is of type `Integer`, it behaves differently and causes an `ArrayIndexOutOfBoundsException`.

With the brackets around `width/gridscale` it is the same term as for the instantiation of the array
`dnasize = (width / gridscale) * (height / gridscale); `

So I think it should be save that way.