Fix docstring returned type issue.

[mairanteodoro]

This PR addresses an error that was caused by an unknown python type provided in the docstring's returned type section.

Checklist

• [ ] added entry in CHANGES.rst (either in Bug Fixes or Changes to API)
• [ ] updated relevant tests
• [X] updated relevant documentation
• [ ] updated relevant milestone(s)
• [X] added relevant label(s)

[codecov[bot]]

Codecov Report

All modified lines are covered by tests :white_check_mark:

Files	Coverage Δ
src/stcal/alignment/util.py	81.22% <100.00%> (ø)

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