PSI typo in derived units

[kurtronshausen]

I just noticed that there's a typo in name for psi. It's "pounds per sqare inch" instead of "pounds per square inch"

[joshwlewis]

Thanks so much for pointing this out. As mentioned here, I don't think Unitwise is the right place to fix this. We should probably push for this fix in UCUM itself, which would also fix it for Unitwise.