DOC: Change sampling kwarg name to spacing in marching_cubes

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 to `spacing`.
DOC: Change sampling to spacing in skimage.draw.ellipsoid and tests
DOC: Fix capitalization in ellipsoid docstring.
Merge pull request #780 from JDWarner/spacing_marching_cubes

DOC: Change sampling kwarg name to spacing in marching_cubes