Fix wrong hint mirror behaviour in Firefox

Change-Id: I9c6b2ff8e24d52bd6c4b4b7856c65946e25f1261
diff --git a/dev/js/spec/hintSpec.js b/dev/js/spec/hintSpec.js
index f45496c..79726f3 100644
--- a/dev/js/spec/hintSpec.js
+++ b/dev/js/spec/hintSpec.js
@@ -61,7 +61,7 @@
   describe('KorAP.InputField', function () {
     beforeAll(beforeAllFunc);
     afterAll(afterAllFunc);
-    var input;
+    let input;
 
     beforeEach(function () {
       input = document.createElement("input");