Fix unit typo in docs

[larrybradley]

The flux step appears to assume the input units are DN/s, not e/s.

Checklist

• [ ] added entry in CHANGES.rst under the corresponding subsection
• [ ] updated relevant tests
• [ ] updated relevant documentation
• [ ] updated relevant milestone(s)
• [ ] added relevant label(s)
• [ ] ran regression tests, post a link to the Jenkins job below. How to run regression tests on a PR

[codecov[bot]]

Codecov Report

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

Project coverage is 77.36%. Comparing base (2df177e) to head (e232ba3). Report is 7 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1200 +/- ## ========================================== + Coverage 77.15% 77.36% +0.20% ========================================== Files 107 109 +2 Lines 7101 7295 +194 ========================================== + Hits 5479 5644 +165 - Misses 1622 1651 +29 ``` | [Flag](https://app.codecov.io/gh/spacetelescope/romancal/pull/1200/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=spacetelescope) | Coverage Δ | | *Carryforward flag | |---|---|---|---| | [nightly](https://app.codecov.io/gh/spacetelescope/romancal/pull/1200/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=spacetelescope) | `62.80% <ø> (ø)` | | Carriedforward from [e2eddc1](https://app.codecov.io/gh/spacetelescope/romancal/commit/e2eddc15e02b2ccc9c5f26ffcf5fbe0a558961c4?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=spacetelescope) | *This pull request uses carry forward flags. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=spacetelescope) to find out more.

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