There are a large number of plural/possessive confusions in game text, not sure if they've all been reported. The most frequent one I have seen is improvements, spells or other improvements to a city; in every single case, the text from the improvements/spell/etc. incorrectly uses the plural of city ("cities") when they affect the city and should be possessive ("city's").
A property of something, such as the food output of a city, it is possessive (id est: "a city's food output" -- food output from a city). Plural ("cities") is not for the property of anything, merely a group of more than one thing. There may be, on occasion, a need for both or a word properly ends in an 's' even for a singular item; in most cases of a word ending in s, possessive is implemented by simply adding an apostrophe without the customary extra 's,' since that would be awkward ("Improves all cities' food output").