Isn't the story that the script he was given was to say "one small step for a man", but on the day he flubbed his line and actually did say "one small step for man"
NASA trusted those guys enough to give them carte blanche with stuff like that, which is why Pete Conrad said "Whoopie! Man, that may have been a small one for Neil, but that's a long one for me" in order to win a bet (and because it was funny).
