DOC: Ryden04 ellipticity missing `returns` docstring

[Lucia-Fonseca]

Describe the bug
The skypy.galaxies.morphology.ryden04_ellipticities function is missing the Returns section in the docstring.

To Reproduce
Steps to reproduce the behavior:

1. Go to skypy.galaxies.morphology.ryden04_ellipticities
2. See Returns section missing

Expected behavior
The Returns section of the docstring should be included. Suggestion:

Returns
--------
ellipticity: (ng,) array_like
    Ellipticity distribution sampled from the Ryden model [1].