Fix event urls in ical feed

[saerdnaer]

The iCal feeds like https://fosdem.org/2026/schedule/track/dev-random.ics currently contain a broken URL:

Current: https:/fosdem.org/2026/schedule/2026/schedule/event/ZUVAZD-four_year_bus/
Should be: https:/fosdem.org/2026/schedule/event/ZUVAZD-four_year_bus/

BEGIN:VEVENT
METHOD:PUBLISH
UID:9940@fosdem-2026@fosdem.org
TZID:Europe-Brussels
DTSTART:20260201T092000
DTEND:20260201T093500
SUMMARY:Four Year Bus

CLASS:PUBLIC
STATUS:CONFIRMED
CATEGORIES:/dev/random
URL:https:/fosdem.org/2026/schedule/2026/schedule/event/ZUVAZD-four_year_bus/
LOCATION:H.2215 (Ferrer)

ATTENDEE;ROLE=REQ-PARTICIPANT;CUTYPE=INDIVIDUAL;CN="Ruán Murgatroyd":invalid:nomail

END:VEVENT