Fix code in Tree docs (set parent to weak)

Author: chrisamanse

Tree/README.markdown

@@ -26,7 +26,7 @@ Here's a basic implementation in Swift:
 public class TreeNode<T> {
   public var value: T
 
-  public var parent: TreeNode?
+  public weak var parent: TreeNode?
   public var children = [TreeNode<T>]()
 
   public init(value: T) {