correct commands to launch subsequent notebook

[shanjon]

the final cell in quickstart.ipynb, create-models.ipynb and model-types.ipynb is intended to launch the subsequent notebook in the series, but the following changes need to be made for it to work:

• notebook is imported from app.py, not notebookapp.py -- notebookapp.py doesn't appear to exist
• list_running_servers method is defined in serverapp.py, not app.py -- method does not exist in app.py

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