Fix astropy default_cosmology in power spectrum documentation plot

[rrjbca]

Description

Update the plot in the power spectrum documentation to match the new function call API.

Note this still fails as there is an independent issue with the halofit functions; See readthedocs build error: AttributeError: 'functools.partial' object has no attribute '__name__'. #527 addresses this second issue, but this PR should be merged first.

Checklist

• [x] Follow the Contributor Guidelines
• [ ] Write unit tests
• [ ] Write documentation strings
• [x] Assign someone from your working team to review this pull request
• [x] Assign someone from the infrastructure team to review this pull request