Small typo in the itensor Documentation

[Zhen_H]

Dear ITensors authors,

Thank you very much for this wonderful tool. I happened to find a harmless typo in the documentation, where the Down state is refered to as the S_z=0 state. Not sure if here is the right place to post it.

All the Best,
Zhen

[miles]

Thanks! This is very helpful and I just created a PR that fixes this mistake.