Fix docs to specify correct set of catalog file types

[bhazelton]

Description

Fix the docs to correctly specify the allowed file types for sky model catalogs.

Motivation and Context

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] Reference simulation update or replacement
• [x] Documentation change (documentation changes only)
• [ ] Version change
• [ ] Build or continuous integration change

Checklist:

For all pull requests:

• [x] I have read the contribution guide.
• [x] My code follows the code style of this project.

Documentation change checklist:

• [x] Any updated docstrings use the numpy docstring format.
• [ ] If this is a significant change to the readme or other docs, I have checked that they are rendered properly on ReadTheDocs. (you may need help to get this branch to build on RTD, just ask!)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.95%. Comparing base (54e9cca) to head (a990526).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #463 +/- ## ======================================= Coverage 99.95% 99.95% ======================================= Files 12 12 Lines 2161 2161 ======================================= Hits 2160 2160 Misses 1 1 ```

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