Fix boilerplate not including core css media type

Author: imakewebthings

boilerplate.html

@@ -8,7 +8,7 @@
   <title>Your deck.js Presentation</title>
 
   <!-- Required stylesheet -->
-  <link rel="stylesheet" href="core/deck.core.css">
+  <link rel="stylesheet" media="screen" href="core/deck.core.css">
 
   <!-- Extension CSS files go here. Remove or add as needed. -->
   <link rel="stylesheet" media="screen" href="extensions/goto/deck.goto.css">