Fix template instance variable error

Received the following error without making this patch to the template.

Aug  8 23:01:34 puppet puppet-master[4575]: /etc/puppet/modules/nrpe/templates/nrpe.cfg.erb:249: formal argument cannot be an instance variable
Aug  8 23:01:34 puppet puppet-master[4575]: ;  @service_check_command.each_pair do |@name, @command|