Fix train_evo2 constant steps description (#1094)

### Description

Fixes the `--help` description of `--constant-steps` in `train_evo2`
which says the constant steps happen before cosine annealing but they
happen afterwards. You can see the implementation
[here](https://github.com/NVIDIA-NeMo/NeMo/blob/main/nemo/lightning/pytorch/optim/lr_scheduler.py)
and below is an example of what happens when using the default 80k
constant steps:

<img width="1400" height="725" alt="image"
src="https://github.com/user-attachments/assets/a5713781-46eb-4c37-a8cc-60c8b1d7efaf"
/>

### Type of changes

<!-- Mark the relevant option with an [x] -->

- [ ] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Refactor
- [x] Documentation update
- [ ] Other (please describe):

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### Usage

<!--- How does a user interact with the changed code -->

```python
train_evo2 --help | grep --after-context=3 "^  --constant-steps"
```

### Pre-submit Checklist

<!--- Ensure all items are completed before submitting -->

- [x] I have tested these changes locally
- [x] I have updated the documentation accordingly
- [x] I have added/updated tests as needed
- [x] All existing tests pass successfully


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Documentation**
* Clarified CLI help text for the --constant-steps option to say
“constant at minimum after annealing” for improved accuracy.
* Explicitly notes this is a wording change only — no defaults,
functionality, or runtime behavior were modified.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: jannis-baum

sub-packages/bionemo-evo2/src/bionemo/evo2/run/train.py

@@ -274,7 +274,7 @@ def parse_args(args: Optional[List[str]] = None) -> argparse.Namespace:
     parser.add_argument(
         "--constant-steps",
         type=int,
-        help="Number of steps to keep the learning rate constant before annealing. This controls the "
+        help="Number of steps to keep the learning rate constant at minimum after annealing. This controls the "
         "shape of the learning rate curve.",
         default=80000,
     )