Error in Ch4 .argmax() function of Simulating Many Random Walks at Once

Hello,

In the last example of Ch 4, Simulating Many Random Walks at Once, I think the function `argmax` only returns the "first crossing time" instead of the "crossing times":
```python
crossing_times = (np.abs(walks[hits30]) >= 30).argmax(axis=1)
```

Regards,
Zhenshan