Fix typo in roberta-base-squad2-v2 model card
# What does this PR do?
Simply adding `-v2` for Haystack API model loading. Furthermore, I've also changed `model` in `model_name_or_path` due to breaking change in Haystack (https://github.com/deepset-ai/haystack/pull/510).

## Who can review?

 Model Cards: @julien-c 