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://cloud.githubusercontent.com/assets/157609/4379156/3104408a-435e-11e4-99c0-fb5a6616ec53.png).

My apologies.