[Bug]: Typo in regt_buying_power calculation description in TradeAccount model

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Current Behavior

The regt_buying_power field in the TradeAccount class (alpaca.trading.models.TradeAccount) contains a typo in its calculation description.

Current documentation text:

Code
User’s buying power under Regulation T (excess equity - (equity - margin value) - * margin multiplier)
Issue:
There is an extra - before the * margin multiplier, which makes the formula unclear and potentially misleading.

### Expected Behavior

The description should correctly reflect the intended calculation without the stray - character.

### SDK Version I encountered this issue in

0.43.2

### Steps To Reproduce

```markdown
References
Alpaca Python SDK API Reference (https://alpaca.markets/sdks/python/api_reference/trading/models.html#alpaca.trading.models.TradeAccount.regt_buying_power)
```

### Filled out the Steps to Reproduce section?

- [x] I have entered valid steps to reproduce my issue or have attached a minimally reproducible case in code that shows my issue happening; and understand that without this my issue will be flagged as invalid and closed after 30 days.

### Anything else?

alpaca-py GitHub repository (https://github.com/alpacahq/alpaca-py/tree/master/docs/api_reference/trading)