I am not an expert on guitar repair, but I've heard of emery boards being used for exactly such a job, alternatively some very fine sandpaper. Very gently, though, file off too much and you've got a much worse problem on your hands.

Before you do that, try and work out what has changed to make it start to buzz- I doubt your 6th fret has grown! It's possible the neck is bending back slightly (have you changed to lighter strings recently?). That would mean your strings are closer to the fretboard, in which case a slight turn of the truss rod (if your travel guitar has one) might suffice to stop it.