suzerain: #3070 fix sign in NRBC write up

I made a sign mistake converting from physical to Fourier space.
This commit propagates the correct, negative sign on the i k_x
and i k_z terms through their final appearance in the model document.



git-svn-id: https://svn.ices.utexas.edu/repos/pecos/suzerain/trunk@44404 0bd85f0b-c151-43b9-a1e9-27231104551f