Pass density as numpy array on prism_layer example

[santisoler]

Patch the prism_layer example so the density of the prisms is passed as
a Numpy array instead as a DataArray. This solves an xarray error when
building the docs. This is just a patch, although the problem must be solved
upstream in Verde.

Related to fatiando/verde#330

Reminders:

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst and the base __init__.py file for the package.
• Write detailed docstrings for all functions/classes/methods. It often helps to design better code if you write the docstrings first.
• If adding new functionality, add an example to the docstring, gallery, and/or tutorials.
• Add your full name, affiliation, and ORCID (optional) to the AUTHORS.md file (if you haven't already) in case you'd like to be listed as an author on the Zenodo archive of the next release.

[santisoler]

This PR is a cheap solution that is not solving anything in the end. I'm closing it.

[santisoler]

See #243 for a good candidate of a proper solution to the issue