Revert url to 'main' from '3.13'

The branch in the source links in the revised help.html
become '3.13' instead of 'main' because the html
was generated from an edited 3.13 version of idle.rst
instead being backported from main.  This reversion
avoids the possibility of future backport merge conflicts.

The version makes no difference to the Help/IDLE doc display as
the url is currently neither displayed nor linked to the link title
-- 'Lib/idlelib'.  If we do make the links live, we can use
sys.version to replace 'main' in backported versions.

Author: terryjreedy