Merge pull request #9427 from saiwang/fix-example-snippet

fix mixin code example

css.html

@@ -2075,7 +2075,7 @@ For example, <code>&lt;section&gt;</code> should be wrapped as inline.
 {% endhighlight %}
 {% highlight css %}
 // Mixin
-.clearfix {
+.clearfix() {
   &:before,
   &:after {
     content: " ";