Update reverse function in LinkedList

Add `tail = node` to update tail.

Author: khuong291

Linked List/README.markdown

@@ -451,6 +451,7 @@ How about reversing a list, so that the head becomes the tail and vice versa? Th
 ```swift
   public func reverse() {
     var node = head
+    tail = node // If you had a tail pointer
     while let currentNode = node {
       node = currentNode.next
       swap(&currentNode.next, &currentNode.previous)