Adding a note to the change_resource_record_sets method to avoid errors

To avoid these errors

     AWS::Route53::Errors::InvalidInput:
       Invalid XML ; cvc-complex-type.2.4.a: Invalid content was found starting with element 'ResourceRecords'. One of '{"https://route53.amazonaws.com/doc/2012-02-29/":SetIdentifier, "https://route53.amazonaws.com/doc/2012-02-29/":AliasTarget, "https://route53.amazonaws.com/doc/2012-02-29/":TTL}' is expected.

it would be nice to have a little note in the comments, so this adds it.