Add functions dependency (#906)

Author: samtstern

fulltext-search-firestore/public/index.html

@@ -59,6 +59,7 @@ <h3>Results</h3>
         <script src="/__/firebase/8.10.0/firebase-app.js"></script>
         <script src="/__/firebase/8.10.0/firebase-auth.js"></script>
         <script src="/__/firebase/8.10.0/firebase-firestore.js"></script>
+        <script src="/__/firebase/8.10.0/firebase-firestore.js"></script>
 
         <script src="/__/firebase/init.js"></script>