fix: `impl std::fmt::Display for IndexTuple` is incorrect for normal without texture coordinate

[ishbosamiya]

When using IndexTuple(0, None, Some(0)), the output should be 1//1 but the output was 1/1.

1/1 implies position index followed by texture coordinate index where as 1//1 implies position index followed by no texture coordinate index followed by normal index which is the expected output.

Reference

• https://en.wikipedia.org/wiki/Wavefront_.obj_file#Vertex_normal_indices_without_texture_coordinate_indices