Fixed comment

 /**
 -* webvtt_next_utf8
  *
 -* move the 'prev' pointer to the beginning of the previous utf8
  * character
  * sequence.
  */
 /**
 +* webvtt_prev_utf8
  *
 +* move the 'end' pointer to the beginning of the previous utf8
  * character
  * sequence.
  */