Add missing gold_passages_retrieved function

[tom-doerr]

The function gold_passages_retrieved is currently missing, without it the tutorial doesn't work.
I copied the function from

dspy/tests/examples/test_baleen.py

Line 98 in eab1324

def gold_passages_retrieved(example, pred, trace=None):

I tested the code with this new function and I get similar numbers as in the tutorial

[arnavsinghvi11]

Thanks @tom-doerr !