Added the word "for"

The intent was to make the direction more readable and remove ambiguity.

Author: richnoah

selectors-4/Overview.bs

@@ -820,7 +820,7 @@ Syntax</h4>
 	user agents must also accept the previous one-colon notation
 	for the Level 1 &amp; 2 pseudo-elements
 	(''::before'', ''::after'', ''::first-line'', and ''::first-letter'').
-	This compatibility notation is not allowed any other <a>pseudo-elements</a>.
+	This compatibility notation is not allowed for any other <a>pseudo-elements</a>.
 	However, as this syntax is deprecated,
 	authors should use the Level 3+ double-colon syntax for these <a>pseudo-elements</a>.