fix Sass filter docstring to use load_paths, not includes_dir

The kwarg includes_dir was deprecated in
744fbfc91d1cee53c83ace35fdf0c4654da7465a but the docstring was not updated
accordingly.
"Merge pull request #358 from aconrad/fix-docstring-about-include_dir\n\nfix Sass filter docstring to use load_paths, not includes_dir"