should have used anchor, not oid

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@30935 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ext/psych/lib/psych/visitors/yaml_tree.rb

@@ -74,8 +74,8 @@ module Psych
           oid         = target.object_id
           node        = @st[oid]
           anchor      = oid.to_s
-          node.anchor = oid
-          return @emitter.alias oid
+          node.anchor = anchor
+          return @emitter.alias anchor
         end
 
         if target.respond_to?(:to_yaml)