Fix ServerValue.TIMESTAMP usage in docs (#742)

Update the docs with correct reference to ServerValue.TIMESTAMP

Author: leoruhland

docs/guide/synchronized-arrays.md

@@ -160,7 +160,7 @@ app.controller("ChatCtrl", ["$scope", "chatMessages",
       $scope.messages.$add({
         from: $scope.user,
         content: $scope.message,
-        timestamp: Firebase.ServerValue.TIMESTAMP
+        timestamp: firebase.database.ServerValue.TIMESTAMP
       });
 
       $scope.message = "";
@@ -172,7 +172,7 @@ app.controller("ChatCtrl", ["$scope", "chatMessages",
         $scope.messages.$add({
           from: "Uri",
           content: "Hello!",
-          timestamp: Firebase.ServerValue.TIMESTAMP
+          timestamp: firebase.database.ServerValue.TIMESTAMP
         });
       }
     });