Use correct icon name

https://github.com/feross/webtorrent-desktop/issues/467

The icon name is "closed_caption", not "closed_captions"
http://jsbin.com/fenejob/1/edit?html,output

The extra space we're seeing is the icon font rendering the 's', which
renders as nothing.

renderer/views/player.js

@@ -363,10 +363,10 @@ function renderPlayerControls (state) {
   if (state.playing.type === 'video') {
     // show closed captions icon
     elements.push(hx`
-      <i.icon.closed-captions.float-right
+      <i.icon.closed-caption.float-right
         class=${captionsClass}
         onclick=${handleSubtitles}>
-        closed_captions
+        closed_caption
       </i>
     `)
   }