Twenty Seventeen: Optimize theme screenshot.

This further reduces image size by ~1.5 KB after running `npm run grunt precommit:image`.

Follow-up to [38833], [57322].

See #58996.

git-svn-id: https://develop.svn.wordpress.org/trunk@58442 602fd350-edb4-49c9-b593-d223f7449a82