Add missing references to calspec solar spectrum.

[mkelley]

Update calib docstring and calib.solar_sources.sources to include calspec spectrum. Documentation (docs/sbpy/calib.rst) is already up to date.

[codecov[bot]]

Codecov Report

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

Project coverage is 77.52%. Comparing base (1315915) to head (8668376). Report is 24 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #387 +/- ## ======================================= Coverage 77.52% 77.52% ======================================= Files 81 81 Lines 7096 7096 ======================================= Hits 5501 5501 Misses 1595 1595 ```

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