Replace dead reference to sample dataset in Jupyter notebook demo

[henrykrumb]

The example Jupyter notebook inside the main project directory (main branch) fails when trying to load the sample dataset.

This PR replaces the sample dataset with a different sample dataset from 3D Slicer as a temporary fix. Unlike the previous dataset, which doesn't seem to exist on the server anymore, the 3D Slicer dataset doesn't feature a KWire which would be interesting for testing the "insert instrument" functionality.

Resolves part of issue #94 .