Mention that SST is called tos in the model

[ejlevine]

I'm submitting this, my 1st ProjectPythia PR, per the instructions in the Pythia Foundations Book, in the section "Contribute to Project Pythia via Github".

https://foundations.projectpythia.org/foundations/github/contribute-to-pythia.html

BTW, this sections says "Let’s add the content tag and open this one as a draft for now", but I don't see a way to add tags in github.

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[jukent]

Thanks for your contribution @ejlevine ! I will wait for the checks to pass before approving.