[cssom-1] Grammar error

Author: fantasai

cssom-1/Overview.bs

@@ -980,7 +980,7 @@ The <dfn method for=CSSStyleSheet>addRule(<var>selector</var>, <var>block</var>,
 
 <p class=advisement>
   Authors should not use these members
-  and instead use and teach the standard {{CSSStyleSheet}} interface defined earlier,
+  and should instead use and teach the standard {{CSSStyleSheet}} interface defined earlier,
   which is consistent with {{CSSGroupingRule}}.
 </p>