fix description on LabQ2sRGB
fix description on LabQ2sRGB

thanks dg1727

see https://github.com/jcupitt/libvips/issues/272

lovell/libvips,leiyangyou/libvips,mcanthony/libvips,jcupitt/libvips,jcupitt/libvips,lovell/libvips,jcupitt/libvips,lovell/libvips,jcupitt/libvips,leiyangyou/libvips,lovell/libvips,mcanthony/libvips,leiyangyou/libvips,mcanthony/libvips,leiyangyou/libvips,mcanthony/libvips