[css-conditional] Fix one ignored vars warning by using variable again.

Author: dbaron

css-conditional/Overview.bs

@@ -861,7 +861,7 @@ interface CSSGroupingRule : CSSRule {
     It must throw INDEX_SIZE_ERR
     if index is greater than <code>cssRules.length</code>.
     It must throw SYNTAX_ERR
-    if the rule has a syntax error and is unparseable;
+    if <var>rule</var> has a syntax error and is unparseable;
     this does not include syntax errors handled by error handling rules
     for constructs inside of the rule,
     but this does include cases where the string given

css-conditional/Overview.html

@@ -774,7 +774,7 @@ <h3 class="heading settled" data-level="7.2" id="the-cssgroupingrule-interface">
     It must throw INDEX_SIZE_ERR
     if index is greater than <code>cssRules.length</code>.
     It must throw SYNTAX_ERR
-    if the rule has a syntax error and is unparseable;
+    if <var>rule</var> has a syntax error and is unparseable;
     this does not include syntax errors handled by error handling rules
     for constructs inside of the rule,
     but this does include cases where the string given