fix rubocop violation

Author: pcai

lib/winrm/shells/base.rb

@@ -187,6 +187,7 @@ def remove_finalizer
       def self.finalize(connection_opts, transport, shell_id)
         proc { Thread.new { close_shell(connection_opts, transport, shell_id) } }
       end
+      private_class_method :finalize
     end
   end
 end