docs: fix sns default value

[antongolub]

CC @1000ch

After v7.0.0 update, another image appeared with the same options.

antongolub-forks@90977fe brings cwebp-bin@6 which uses imagemin/cwebp-bin@7f49333 cwebp-1.2.1 which now uses sns=50 as default

So the docs should be updated.

[antongolub]

or maybe it's better to restore the prev default?

if (options.sns) {
  args.push('-sns', options.sns);
}

→

args.push('-sns', options.sns !== undefined ?  options.sns : 80);