Update G042.md with P bool command

[classicrocker883]

G42 has this code

    #if HAS_PROBE_XY_OFFSET
      if (parser.boolval('P')) {
        if (hasI) destination.x -= probe.offset_xy.x;
        if (hasJ) destination.y -= probe.offset_xy.y;
      }
    #endif

so this documentation should be updated to include this bool type parameter.