fix minor issues in `main.f90`

[Pedro Costa]

1. if(myid == 0) write(stderr,*) 'ERROR: implicit diffusion not yet supported with "_FFT_USE_SLICED_PENCILS".' the first instance of this error message needs fixing.

2. the following too:

#elif  _FFT_Z
                    dyf(lo(2)) == dzf(lo(3))

[Pedro Costa]

fixed by 08e2ddd