Fix formatting of start dates

Previously, a timestamp instead of a simple date had been used instead.
This threw off the https://github.com/emersion/go-ical parser.

sphinx-termine/termine.py

@@ -497,7 +497,7 @@ def generate_ics(events, domain="sphinx-termine", base_url="https://example.com"
                 # TODO use the file modification timestamp of the defining file
                 "DTSTAMP:", _format_timestamp(datetime.datetime.now()), "\r\n"
 
-                "DTSTART;VALUE=DATE:", _format_timestamp(event.start), "\r\n"
+                "DTSTART;VALUE=DATE:", _format_date(event.start), "\r\n"
                 "DTEND;VALUE=DATE:", _format_date(end), "\r\n",
                 _limit_line_length("SUMMARY:" + _quote(event.title)) + "\r\n"])
         if event.link: