fix typo in VIPS_ARG_POINTER
fix typo in VIPS_ARG_POINTER

thanks  Fingolfin1196 !

see https://github.com/libvips/libvips/issues/1187

lovell/libvips,jcupitt/libvips,lovell/libvips,jcupitt/libvips,jcupitt/libvips,jcupitt/libvips,lovell/libvips,lovell/libvips