Add example to the draft - ptr

In section 5.5 "ptr"

"This mechanism matches if the <target-name> is an ancestor of a
   validated hostname, or if the <target-name> and a validated hostname
   are the same.  For example: "mail.example.com" is within the domain
   "example.com", but "mail.bad-example.com" is not.  If a validated
   hostname is the <target-name>, a match results."

Maybe a better (or additional) example would be:
For example: "mail.employees.example.com" is within the domain
   "employees.example.com", but "mail.example.com" is not.  If a validated
   hostname is the <target-name>, a match results."

Thanks,
Guy