Fix wording of example 12 (#511)

closes #510

index.html

@@ -431,7 +431,7 @@ <h3>Adhere to the rules of HTML</h3>
           ... 
           &lt;p>&lt;/p>
 
-          &lt;!-- Use a span are allowed in p elements! -->
+          &lt;!-- Instead of a div, use a span. Spans are allowed descendants of p elements! -->
           &lt;p>
             ... &lt;span role=link tabindex=0>...&lt;/span> ...
           &lt;/p>