[docs] Update link in working_with_auxiliary_data.md

[lgabs]

in working_with_auxiliary_data.md there is a broken link, probably it would point to the module's implementation.

[darrylong]

Thank you for contributing to Cornac!

Instead of using a specific version of the documentation, could we use the 'stable' branch instead?

That will be:
https://cornac.readthedocs.io/en/stable/api_ref/data.html#module-cornac.data.modality