UX: fix typo in lazyYT.css

For some reason my previous PR https://github.com/discourse/discourse/pull/2819 had wrong padding (twice as big).
Fixing it  (here and upstream at https://github.com/tylerpearson/lazyYT/pull/11) now so that it will look as  [expected](https://github.com/discourse/discourse/pull/2819#issue-43679040).

My apologies.