This is actually an interesting legal issue IMO, because it pits one extreme - the utter harmlessness of the misplaced word, the clear intent to have taken the oath correctly - against the most important legal document we have haivng a black and white clear requirement, where a precedent to cut corners once opens the door to cut them again, making it very important the letter of the law be followed.
If you cut the corner here, what could be next - someone 34 running for president because he is 35 if you measure his age beginning at conception?
Defining someone born elsewhere as a 'natural born American' because they were born naturally, and they're an American?
As I've pondred this, I have to withdraw my mockery of those who would have tried to make an issue of it. I think their case is too legitimate to do that.
Barack handled this situation just right IMO, by quickly acting to get the oath done correctly.