Dialog demo: Remove reference to obsolete overlay option.

Author: scottgonzalez

demos/dialog/modal.html

@@ -53,7 +53,7 @@
 
 <div class="demo-description">
 
-<p>A modal dialog prevents the user from interacting with the rest of the page until it is closed.  To add a semi-transparent layer that dims out the page content behind the dialog, set the background color and opacity of the <code>overlay</code> option.</p>
+<p>A modal dialog prevents the user from interacting with the rest of the page until it is closed.</p>
 
 </div><!-- End demo-description -->