[htmlmixed mode] Fix example on demo page

mode/htmlmixed/index.html

@@ -87,7 +87,7 @@
     <pre>var myModeSpec = {
   name: "htmlmixed",
   tags: {
-    style: [["type", /^text/(x-)?scss$/, "text/x-scss"],
+    style: [["type", /^text\/(x-)?scss$/, "text/x-scss"],
             [null, null, "css"]],
     custom: [[null, null, "customMode"]]
   }