docs/website: fixing mixing content error

Firefox recently activated mixing content mode protection
(http elements in an https page). This patch changes
the external slides service to https.

Signed-off-by: Angelo Compagnucci <angelo.compagnucci@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

docs/website/js/buildroot.js

@@ -78,7 +78,7 @@ jQuery(document).ready(function($) {
     url = url.split(/[\\/]/).pop()
     $('.nav a[href="/' + url + '"]').parent().addClass('active');
 
-    $('#slides').html('<iframe src="http://docs.google.com/gview?url=http://free-electrons.com/doc/training/buildroot/buildroot-slides.pdf&embedded=true" style="position:absolute; width:100%; height:100%; top:0; left:0;" frameborder="0"></iframe>')
+    $('#slides').html('<iframe src="https://docs.google.com/gview?url=http://free-electrons.com/doc/training/buildroot/buildroot-slides.pdf&embedded=true" style="position:absolute; width:100%; height:100%; top:0; left:0;" frameborder="0"></iframe>')
 });
 
 function showTooltip(elem, msg) {