You can hear the long pause between the words "one" and "giant" as Neil Armstrong realizes he flubbed his one line that he had rehearsed over and over again in front of the mirror.
Seems like I read something a few years back, stating that analysis of the audio indicates Armstrong actually said "a man", but it wasn't originally transmitted and/or received and/or audible to the same extent that his other words were.
Who knows (or cares)? It's not like he went there, just to read some previously prepared and officially approved comments on the radio...
He should have skipped all that high-toned stuff and yelled out “Hot damn, we made it, and sc&@# you, Brezhnev!”