propagation: fix regression in angular spectrum transfer function impl

[mpetroff]

I came across a data centering regression in the current master branch with regard to Wavefront.free_space. The following code results in decentered output in af89c94d500a274eda664188ddb97fcae30c6ac5 but appears fine in v0.21:

import prysm.coordinates
import prysm.geometry
import prysm.propagation

diameter = 550 # mm
xi, eta = prysm.coordinates.make_xy_grid(256, diameter=diameter)
r, t = prysm.coordinates.cart_to_polar(xi, eta)
A = prysm.geometry.truecircle(diameter / 2, r)
dx = xi[0, 1] - xi[0, 0]
wf = prysm.propagation.Wavefront.from_amp_and_phase(A, None, 2e3, dx)
wf.free_space(dz=1000).intensity.plot2d()

Further investigation isolated this issue to typos in the optimization introduced in 61f7ee4f10754d1cb8eae90a019c55256c19ffa9. This PR fixes the regression.

[brandondube]

Thanks for the pull request, and laser eyes to find the typo :)