[chore] Include respec using defer attribute

Attempt to fix tobie/pr-preview#20.

Author: mgiuca

index.html

@@ -6,7 +6,7 @@
       Web Share Target API
     </title>
     <script src="https://www.w3.org/Tools/respec/respec-w3c-common" class=
-    "remove"></script>
+    "remove" defer></script>
     <script class='remove'>
       var respecConfig = {
         specStatus: "CG-DRAFT",