return of nnPU loss function may be mistaken

In 'loss.py' , line 37-38

```
        if negative_risk < -self.beta and self.nnPU:
            return -self.gamma * negative_risk
```

            
Is this correct?

I think the following is correct.

```
        if negative_risk < -self.beta and self.nnPU:
            return positive_risk
```
