qga/qapi-schema: Tidy up documentation of guest-fsfreeze-status

Delete "error state indicates", because it doesn't make sense.
I suspect it was an accident.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Konstantin Kostiuk <kkostiuk@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20230425064223.820979-2-armbru@redhat.com>