Fix `disag_model` example

[simon-smart88]

rnorm(1, ifelse(x %% n_pixels_per_side != 0, x %% n_pixels_per_side, n_pixels_per_side), 3))}

should be

rnorm(1, ifelse(x %% n_pixels_per_side != 0, x %% n_pixels_per_side, n_pixels_per_side), 3)})