(Not the nominator) While the actual word "Jotnar" may not occur in the MCU (I can't recall for certain) the word "Jotun" definitely does, and the Wikipedia page for Jötunn (spelling varies from place to place but definitely the same Norse mythological figure that the MCU is drawing from) lists Jötnar as the plural. (Again, spelling varies from place to place so the umlaut doesn't seem to be required.)