Fix import path for html-tag.

Author: aomarks

polymer.html

@@ -20,7 +20,7 @@
 <!-- bc behaviors -->
 <link rel="import" href="lib/legacy/mutable-data-behavior.html">
 <!-- import html-tag to export html -->
-<link rel="import" href="lib/util/html-tag.html">
+<link rel="import" href="lib/utils/html-tag.html">
 <script>
   // bc
   Polymer.Base = Polymer.LegacyElementMixin(HTMLElement).prototype;