fix broken iframe in backgrounds example

test/examples/slide-backgrounds.html

@@ -93,7 +93,7 @@
 					<h2>Video background</h2>
 				</section>
 
-				<section data-background-iframe="https://slides.com">
+				<section data-background-iframe="https://slides.com/news/make-better-presentations/embed?style=hidden&autoSlide=4000">
 					<h2>Iframe background</h2>
 				</section>