Update index.html

Added https prefix to slideshare iframe link, without this I got a 'not found' error when opening the demo locally.

index.html

@@ -325,7 +325,7 @@ function linkify( selector ) {
 				<section>
 					<h2>Export to PDF</h2>
 					<p>Presentations can be <a href="https://github.com/hakimel/reveal.js#pdf-export">exported to PDF</a>, here's an example:</p>
-					<iframe src="//www.slideshare.net/slideshow/embed_code/42840540" width="445" height="355" frameborder="0" marginwidth="0" marginheight="0" scrolling="no" style="border:3px solid #666; margin-bottom:5px; max-width: 100%;" allowfullscreen> </iframe>
+					<iframe src="https://www.slideshare.net/slideshow/embed_code/42840540" width="445" height="355" frameborder="0" marginwidth="0" marginheight="0" scrolling="no" style="border:3px solid #666; margin-bottom:5px; max-width: 100%;" allowfullscreen> </iframe>
 				</section>
 
 				<section>