Neutral and ground wire switched
We can assume that the mistake starts and ends in that box, but there are other considerations. If the outlet is fed from a sub-panel, neutral and ground aren't (or shouldn't be) bonded. If they're also reversed there, reversing the connections at the outlet could be dangerous. 

If you're confident that you're working with conventional wiring served from the primary service panel, go ahead and make the switch. Otherwise, consult a local expert. I'd have a look at a few more outlets in the area to see if there's a trend. 

Also be aware that if you're working with a 15A circuit, the microwave likely consumes all available current. You won't have any overhead for other devices without causing nuisance trips.