[css-ruby] Fix typo

Fixes w3c#787.

Author: upsuper

css-ruby/Overview.bs

@@ -527,7 +527,7 @@ Autohiding Base-identical Annotations</h3>
 <!--		-->&lt;ruby>
 <!--		-->  &lt;rb>振&lt;/rb>&lt;rb>り&lt;/rb>&lt;rb>仮&lt;/rb>&lt;rb>名&lt;/rb>
 <!--		-->  &lt;rp>(&lt;/rp>&lt;rt>ふ&lt;/rt>&lt;rt>り&lt;/rt>&lt;rt>が&lt;/rt>&lt;rt>な&lt;/rt>&lt;rp>)&lt;/rp>
-<!--		-->&lt;ruby></pre>
+<!--		-->&lt;/ruby></pre>
 		<p>However, when displayed as ruby, the “り” should be hidden
 		<div class="figure">
 			<p><img src="images/furigana-separate.png"