See update below.
Michelle Nunn is the daughter of former Georgia senator Sam Nunn, and the Democrat is running this November to follow in her father's footsteps. The younger Nunn is a native of Perry, Georgia, but she grew up and went to school in the Washington, D.C., area while her father served in the Senate. That could explain why the Democrat incorrectly identified the state of Georgia's motto. From the Associated Press (emphasis added):
At her stops, Nunn massaged her usual critique of partisan rancor to fit Perdue. “We’re going to give voters something different, as opposed to what they saw the last nine weeks” during the bitter Republican runoff. “Georgia’s motto is ‘Union, Justice, Moderation,’” she reminded her supporters. “I think those are pretty good values.”
The actual motto is "Wisdom, Justice, Moderation." The motto can be found on the state seal and the state flag.
For the open Senate seat in Georgia, Nunn faces Republican David Perdue, who defeated congressman Jack Kingston in the GOP runoff this week.
Update: The AP reporter covering the speech writes in to say his transcription of the Nunn quotation was in error. Upon reviewing his recording, the reporter says that Nunn said the correct motto of "Wisdom, Justice, Moderation."