Change "Leave A Comment" title tag

add_filter( 'comment_form_defaults', 'leave_a_comment_title_tag' );
function leave_a_comment_title_tag( $defaults ){
  $defaults['title_reply_before'] = '<p id="reply-title" class="comment-reply-title">';
  $defaults['title_reply_after'] = '</p>';
  return $defaults;
}