Fixes closing firebase-document

Changes closing from `</firebase-collection>` to `</firebase-document>` to match opening tag.

Author: chuckh

demo/index.html

@@ -46,7 +46,7 @@ <h3>Dinosaur Scores Document</h3>
       <template is="dom-bind">
         <firebase-document
           location="https://dinosaur-facts.firebaseio.com/scores"
-          data="{{dinosaurs}}"></firebase-collection>
+          data="{{dinosaurs}}"></firebase-document>
         <x-pretty-json object=[[dinosaurs]]></x-pretty-json>
       </template>
     </div>