What is an example of a malapropism in "Macbeth"?
There are none in this particular play.
Some may mistakenly take the "riddles" of the witches as such, but they are neither intended nor executed as malapropisms. A further search and some re-reading confirmed my suspicion that this particular literary device was NOT used in Macbeth.
Shakespeare used it masterfully (maybe invented it?) in other plays - notably "Much Ado About Nothing", "King Lear", and "Romeo and Juliette". However, he does not use every available literary device in every play.