[css-values] Fix tense of sentence, since we allow unit algebra now.

Author: tabatkins

css-values-4/Overview.bs

@@ -1849,7 +1849,7 @@ Type Checking</h4>
 	such as in 'opacity',
 	are not <em>combinable</em> with those numbers--
 	''opacity: calc(.25 + 25%)'' is invalid.
-	Allowing this would cause significant problems with "unit algebra"
+	Allowing this causes significant problems with "unit algebra"
 	(allowing multiplication/division of <<dimension>>s),
 	and in every case so far,
 	doesn't provide any new functionality.