Fix Gnus parsing of weekly recurring icalendar events

Fix Gnus parsing of weekly recurring icalendar events

* lisp/gnus/gnus-icalendar.el
(gnus-icalendar-event:recurring-interval): Fix parsing of weekly
recurring events (bug#43669).

Example: in the absence of explicit INTERVAL value in the calendar
event, a weekly event with occurrences scheduled for Mondays and
Wednesdays should receive the default recurring interval of "1" and the
org mode timestamp repeater should be "+1w".

Due to a bug in the current code we receive "WEEKLY" and "+WEEKLYw"
instead. The patch fixes the issue.