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

examples/classic-slides/index.html

@@ -34,7 +34,7 @@
     <link rel="stylesheet" href="../../extras/mermaid/mermaid.forest.css">
 
     <!-- This file contains common styles for example presentations. -->
-    <link href="..\..\css\impress-common.css" rel="stylesheet" />   
+    <link href="../../css/impress-common.css" rel="stylesheet" />
     <!--
         This file contains styles specific for this example presentation. 
     -->