Jupyter Notebook Demo outdated and causes errors

(Amazing project!)

I was trying to work through https://github.com/google/neuroglancer/blob/master/python/examples/jupyter-notebook-demo.ipynb, and discovered that some of the code no longer works. In particular,

```python
with viewer.txn() as s:
s.layers['segmentation'].segments.update([1752, 88847])
s.layers['segmentation'].visible = True
```

causes the following error:

```
---------------------------------------------------------------------------
AttributeError Traceback (most recent call last)
Cell In[13], line 2
1 with viewer.txn() as s:
----> 2 s.layers['segmentation'].segments.update([1752, 88847])
3 s.layers['segmentation'].visible = True

AttributeError: 'VisibleSegments' object has no attribute 'update'
```