Small errors in README and explainer/utils.py

[scarrasco-az]

Dear authors,
Thanks for the hard work and for creating such a valuable resource for the community. While working with the code, I came accross a few issues that you may want to review:

• README typo: instead of explain_model.py it should be explain_counterfactuals.py

• In explainer/utils.py: torchmetrics.Accuracy requires specifying the task (and number of classes when necessary). For context, my PyTorch Lightning version is 1.9.4