Fix YARD warning

 Undocumented Objects:

(in file: lib/rspec/core/example_group.rb)
RSpec::Core::ExampleGroup.initialize

Author: pirj

lib/rspec/core/example_group.rb

@@ -686,6 +686,7 @@ def self.each_instance_variable_for_example(group)
         end
       end
 
+      # @private
       def initialize(inspect_output=nil)
         @__inspect_output = inspect_output || '(no description provided)'
         super() # no args get passed