It's not a wrong grammatical correction though:
it's: short for "it is".
its: possessive version of "it". E.g. its worse -> worst belong to "it"
it's: short for "it is".
its: possessive version of "it". E.g. its worse -> worst belong to "it"
EDIT: The original misguided pedant has now changed "worse" to "worst" in both instances; after downvoting me, of course.
https://www.macmillandictionary.com/dictionary/british/at-it...