Add summarization name mapping for MultiNews
Author: JohnGiorgi
Pull request: yes

# What does this PR do?

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Adds the `text_column` and `summary_column` names to the to the `summarization_name_mapping` dictionary in `run_summarization.py`. This allows a user to use the script with [MultiNews](https://huggingface.co/datasets/multi_news) without having to specify these variables explicitly. Admittedly this is a tiny change but benefits anyone using MultiNews with this script.

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Comments:
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