DOC: Change sampling kwarg name to spacing in marching_cubes

Implementing the last change needed after the discussion in #754, where `spacing` beat out `sampling` as the preferred kwarg for anisotropic datasets.

The preference is to follow VTK's convention for anisotropic rectangularly sampled data, using the keyword `spacing` rather than `sampling`. This change converts the last remaining use of `sampling` in scikit-image (in `skimage.measure.marching_cubes` to the new standard for 0.9+, `spacing`.