Fix error message when using load_from_disk to load DatasetDict
Issue #4594 
Issue: When `datasets.load_from_disk` is wrongly used to load a `DatasetDict`, the error message suggests using `datasets.load_from_disk`, which is the same function that generated the error. 
Fix: The appropriate function which should be suggested instead is `datasets.dataset_dict.load_from_disk`.
Changes: Change the suggestion to say "Please use `datasets.dataset_dict.load_from_disk` instead."