Minor typo in course22 welcome page

[fercrcode]

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As I couldn't find the relevant file to fix and push the changes myself I'll open an issue here.

There is a tiny typo in The software you will be using section of the course's website. The following sentence is missing an is:

The fastai library is one of the most popular libraries for adding this higher-level functionality on top of PyTorch

Cheers and thank you all for the amazing work!