[css-env-1] fix pem typo

[kidonng]

I believe this is a typo, but pardon if it's a misunderstanding :)

[JoshTumath]

Thanks for finding the typo!

The CSSWG actually resolved to no longer pursue pem units in #12380 (comment), so I will close this issue, but we appreciate your contribution!

CC @tabatkins. @keithamus is working on defining the <meta text-scale> tag in #12469, but as editor of the css-env-1 spec, will you still need to remove references to pem in the spec yourself?