Typo in chapter4/4.mdx - Variable and metrics

There is a small typo in chapter4/4.mdx [Variable and metrics](https://huggingface.co/learn/nlp-course/chapter4/4?fw=pt#variable-and-metrics)

I think instead of word **mesuring** it should be **measuring** in below sentence,
Here you should describe the metrics you use for evaluation, and the different factors you are **_mesuring_**.

Let me know if I can raise pull request.
Thanks