"a" is an indefinite article, not a preposition. Prepositions are how a verb relates to an indirect object. "The bunny hopped over the fence, around a tree, and down a hole." The italicized words in this sentence are prepositions. In, on, at, near, etc.. You get the idea...

Anyway, the quote is kinda contradictory without the "a". Is it a small step or a giant leap? Oh, it's a giant leap for mankind, and a small step for a man. Makes so much sense now.