Sorry to be pedantic, but I'm drunk and got annoyed by your contraction use.
WORD + 's = WORD + is.
So, "ad's" = "advertisement is". Which shouldn't make sense. It should be "ads".
Sorry to be pedantic but WORD + ‘s = WORD + is, is not always true. If the word is a singular noun you can add an ‘s to form the possessive form of a singular noun;
Jane’s books