[css-conditional-3] Fix comment syntax in example.

Fixes w3c#3994.

Author: dbaron

css-conditional-3/Overview.bs

@@ -104,10 +104,10 @@ conditions are true.
 
 <div class="example">For example, with this set of nested rules:
 <pre>
-@media print { // rule (1)
+@media print { /* rule (1) */
   /* hide navigation controls when printing */
   #navigation { display: none }
-  @media (max-width: 12cm) { // rule (2)
+  @media (max-width: 12cm) { /* rule (2) */
     /* keep notes in flow when printing to narrow pages */
     .note { float: none }
   }