fix(HLS): Fix detection of WebVTT subtitles in HLS by extension (#4928)

We were seeing uncaught TypeErrors when changing subbtitle
tracks.

Closes #4929

lib/hls/hls_parser.js

@@ -3367,7 +3367,7 @@ shaka.hls.HlsParser.TEXT_EXTENSIONS_TO_MIME_TYPES_ = {
   'm4f': 'application/mp4',
   'cmft': 'application/mp4',
   'vtt': 'text/vtt',
-  'webvtt': 'txt/vtt',
+  'webvtt': 'text/vtt',
   'ttml': 'application/ttml+xml',
 };