yaml_tree.rb: fix warning message

* ext/psych/lib/psych/visitors/yaml_tree.rb: fix warning message.
  Class.create does not exist.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40772 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

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

@@ -51,7 +51,7 @@ module Psych
         return super if emitter && ss && options
 
         if $VERBOSE
-          warn "This API is deprecated, please pass an emitter, scalar scanner, and options or call #{self.class}.create() (#{caller.first})"
+          warn "This API is deprecated, please pass an emitter, scalar scanner, and options or call #{class}.create() (#{caller.first})"
         end
         create emitter, ss
       end