Fix type signature of read_problems function

[Linyxus]

The read_problems function in human_eval.data is actually returning a dictionary, but it is wrongly declared to return a iterable of dictionaries:

def read_problems(evalset_file: str = HUMAN_EVAL) -> Iterable[Dict[str, Dict]]:

This is a simple fix of the typo.

[hauntsaninja]

Thank you!