simulator() docstring names the wrong return key ('responses' vs 'choices') and a nonexistent 'deadline' argument

The `simulator()` docstring (`ssms/basic_simulators/simulator.py`) describes a
return dict with keys `'rts', 'responses', 'metadata'` (lines ~671 and ~690).
The actual key is **`'choices'`** — as used internally at `simulator.py:489`,
`:816`, `:819`.

The same docstring documents a `deadline` parameter (lines ~645-646) that is not
in the signature; deadline models are selected via the `_deadline` model-name
suffix instead.

### Reproducer

```python
import inspect
from ssms.basic_simulators.simulator import simulator

out = simulator(theta=[0.5, 1.2, 0.5, 0.3], model="ddm", n_samples=10)
print("documented keys :", "['rts', 'responses', 'metadata']")
print("actual keys :", sorted(out.keys()))
print("'responses' present?", "responses" in out)
print("'deadline' in signature?", "deadline" in inspect.signature(simulator).parameters)
```

```
documented keys : ['rts', 'responses', 'metadata']
actual keys : ['binned_128', 'binned_256', 'choice_p', 'choice_p_no_omission',
'choices', 'go_p', 'metadata', 'nogo_p', 'omission_p', 'rts']
'responses' present? False
'deadline' in signature? False
```

Documentation-only fix, but it is the first thing a new user reads.

**Version:** ssm-simulators 0.13.2

Open ssms/basic_simulators/simulator.py and inspect the simulator() docstring around lines 645-646, 671, and 690, comparing it with the signature and the actual keys used at lines 489, 816, and 819. Update the documented parameter and return-key descriptions, then verify that the docstring matches the shown signature and simulator output.

Written by the indexing model from the issue text.