The rule hasn't changed. The form 's indicates possession. What has changed is a nasty invention called Spell Check and even worse Grammar Check which allow the lazy person to feel s/he has fixed all the tiddly problems that arise from not learning grammar, punctuation and spelling. I have absolutely no problem with being a total Cranky Pants. For some odd reason, punctuation, grammar and spelling are not drilled in school anymore. Elementary school is the place for this. I believe I learned the entire grammar of the English language in Mrs. Margie Presswood's 7th grade English class in Comanche, Texas. She tried to make it fun, but it was dry, dry, dry. I hated that class and was never so happy to see a school year end. But I learned it and so did 90 percent of my classmates. We knew grammar and punctuation. And then I was a student of Mrs. Mary Brown in Waco, Texas as a senior in her Senior Year Humanities Class. OMG -- She was a slave driver and had no humor at all about poor grammar, etc. By the end of her class, I was not just ready for freshman year of University. I was ready for anything that could be thrown at me in the way of writing assignments.
All that is to say that I am cranky about the lack of precision in the language now. Schools just don't drill or concentrate on 'dull' classes. As much instruction as possible is taught via computer 'game' so that students don't lose interest. My sister, who teaches 2nd grade, assures me that children now can not concentrate on anything for longer than 15-20 minutes.
I believe you are completely justified to correct that announcement envelope, tuck it into a correctly addressed one, and mail it back to the child who graduated college without a basic grade school education.
Sunny on her soapbox