Fix typo of unit name in css-env draft (w3c#13008)

1em didn't make sense with the associated formula. 

The context of the line above makes me think it was meant to be pem.

The definition of pem here fits the line

w3c#10674

Although it also suggests it's going away, so maybe the whole section needs removing.

Author: ccouzens

css-env-1/Overview.bs

@@ -256,7 +256,7 @@ would cause text sizes to double,
 then ''env(preferred-text-scale)'' would resolve to ''2''.
 
 Note: The ''pem'' unit represents this same information;
-''1em'' is exactly equivalent to ''calc(1em * env(preferred-text-scale))''.
+''1pem'' is exactly equivalent to ''calc(1em * env(preferred-text-scale))''.
 When directly sizing things, ''bsem'' is just a more convenient length to use.
 
 <div class=example>