fix: playback rate default text (#4558)

Previously, the default text only said `1` now it says `1x` which matches the look after changing the playback rate.

src/js/control-bar/playback-rate-menu/playback-rate-menu-button.js

@@ -44,7 +44,7 @@ class PlaybackRateMenuButton extends MenuButton {
 
     this.labelEl_ = Dom.createEl('div', {
       className: 'vjs-playback-rate-value',
-      innerHTML: 1.0
+      innerHTML: '1x'
     });
 
     el.appendChild(this.labelEl_);