fixed webvtt to spec, there have to be at least 2 empty lines

templates/chapters.srt

@@ -2,10 +2,13 @@
 WEBVTT
 
 
+
+
 {% for c in chapters|sort(attribute='start') %}
 {{ loop.index }}
 {# time format is 'minutes:seconds.ms' we expect full seconds #}
 {{ vtttime(c['start']) }} --> {{ vtttime(c['end']) }}
 {{ c['text'] }}
 
+{% else $}
 {% endfor %}