Fix mardown syntax and fix indentation to 2spaces tabs (platform independent).

I think we must update the markdown for the descriptor choice.

  SURF_UPRIGHT = 0, // Upright descriptors, not invariant to rotation
  SURF = 1,
  MSURF_UPRIGHT = 2, // Upright descriptors, not invariant to rotation
  MSURF = 3,
  MLDB_UPRIGHT = 4, // Upright descriptors, not invariant to rotation
  MLDB = 5