Tutorial 4 final steps cause an error

This might actually be an issue with the python package, but I'll leave that up to you to decide and open in the python client repo if needed.

The issue is in the final couple of lines of tutorial 4 - I thought I'd add a printline statement to see the output for these two lines...

```
query.add_path_description("ontologyAnnotations.ontologyTerm","Ontology Term")
query.add_path_description("publications","Pub.")
```
But when I try to print we get a badly formed query error.

You can try it out yourself in [this binder instance](https://mybinder.org/v2/gh/yochannah/intermine-ws-python-docs/tutorial-4?filepath=04-tutorial.ipynb)

Error:
```python
WebserviceError: ('There was a problem with our request', 400, 'Bad Request', u"Query does not pass XML validation. cvc-complex-type.3.2.2: Attribute 'path' is not allowed to appear in element 'pathDescription'.\ncvc-complex-type.4: Attribute 'pathString' must appear on element 'pathDescription'.\ncvc-complex-type.3.2.2: Attribute 'path' is not allowed to appear in element 'pathDescription'.\ncvc-complex-type.4: Attribute 'pathString' must appear on element 'pathDescription'.\n")
```

Open 04-tutorial.ipynb on the tutorial-4 branch and run the final cells in the linked Binder instance. Trace the two add_path_description calls and the resulting XML validation error, then determine whether the fix belongs in the tutorial or the Python client; done when the tutorial no longer produces the reported error.

Scritto dal modello di indicizzazione a partire dal testo della issue.