Fix syntax error

Author: spastorino

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

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