Apply suggested changes from review

Co-authored-by: Tab Atkins Jr. <jackalmage@gmail.com>

Author: dlibby-

css-env-1/Overview.bs

@@ -234,7 +234,7 @@ It is only syntax-checked after ''env()'' functions have been [=substituted=].
 	1. If the name provided by the first argument of the ''env()'' function
 		is a recognized [=environment variable=] name, the number of supplied integers
         matches the number of dimensions of the [=environment variable=] referenced
-        by that name, and values of the indices result in a valid value,
+        by that name, and values of the indices correspond to a known sub-value,
 		replace the ''env()'' function by the value of the named [=environment variable=].
 
 	2. Otherwise, if the ''env()'' function has a fallback value as its second argument,