docs(README): update custom events info

Author: joaopaulovieira

README.md

@@ -119,15 +119,16 @@ var player = new Clappr.Player(
     hlsPlayback: {
       ...
       customListeners: [
-        { eventName: 'FRAG_LOADED', callback: (event, data) => { console.log('>>>>>> data: ', data) }, once: true }
+        // "hlsFragLoaded" is the value of HlsjsPlayback.HLSJS.Events.FRAG_LOADED constant.
+        { eventName: 'hlsFragLoaded', callback: (event, data) => { console.log('>>>>>> data: ', data) }, once: true }
       ],
     },
   });
 ```
 
 The listener object parameters are:
 
-* `eventName`: A valid event name of `HLS.JS` [events API](https://github.com/video-dev/hls.js/blob/master/docs/API.md#runtime-events);
+* `eventName`: A valid event name of `hls.js` [events API](https://github.com/video-dev/hls.js/blob/master/docs/API.md#runtime-events);
 * `callback`: The callback that should be called  when the event listened happen.
 * `once`: Flag to configure if the listener needs to be valid just for one time.