Replace backslashes by slashes in href to CSS (#828)

examples/cube/index.html

@@ -5,7 +5,7 @@
     <title>Cube | Explore impress.js in 3D | by Henrik Ingo @henrikingo</title>
     <meta name="description" content="Explore impress.js in 3D" />
     <meta name="author" content="Henrik Ingo" />
-    <link href="..\..\css\impress-common.css" rel="stylesheet" />
+    <link href="../../css/impress-common.css" rel="stylesheet" />
     <link href="css/cube.css" rel="stylesheet" />
 </head>