I doubt you could rotate the plate since the slot for the odometer will be out of whack and the Triumph name will be at an angle an not look good. But you could use whiteout paint and make small stickers with MPH numbers on them and stick them at the corresponding corrected MPH on the speedo. May not look bad.
Since its 10% error, the difference is obvious starting at 40MPH. You then would put a sticker over the 44MPH=40MPH, 55MPH=50MPH, 66MPH=60MPH, 77MPH=70MPH, 88MPH=80MPH, 99MPH =90MPH and so on.
