enrich link broken in docs

[scottkall]

On this page:

https://diffxpy.readthedocs.io/en/latest/tutorials.html#gene-set-enrichment-enrich

The link to this notebook under 'enrich' doesn't work:

https://nbviewer.jupyter.org/github/theislab/diffxpy_tutorials/tree/master/diffxpy_tutorials/enrich/enrich.ipynb

Looks like it should be this instead:

https://nbviewer.jupyter.org/github/theislab/diffxpy_tutorials/blob/master/diffxpy_tutorials/enrich/gene_set_enrichment.ipynb