Update index.js

'var's are replaced with 'let's.

Author: ulasaydin

src/_DataStructures_/LinkedList/reverse-linked-list/index.js

@@ -1,7 +1,7 @@
 function reverseLinkedList(linkedList) {
-    var next = linkedList.getFirst();
-    var current = null;
-    var prev;
+    let next = linkedList.getFirst();
+    let current = null;
+    let prev;
     while(next != null){
         prev = current;
         current = next;
@@ -12,4 +12,4 @@ function reverseLinkedList(linkedList) {
 };
 module.exports = {
     reverseLinkedList,
-  };
+  };