Yeah, ng'e is one of the few errors in the Swahili in this course. It's definitely nge.
Nge is pronounced [ˈŋ̩.gɛ] (two syllables) whereas "ng'e" would be pronounced [ŋɛ] (one syllable) and all nouns, verbs and adjectives need at least two syllables so that they can have a stressed syllable.