ipaddress multicast v6 RFC documentation correction

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2021-09-08.23:02:31.600>
labels = ['3.9']
title = 'ipaddress multicast v6 RFC documentation correction'
updated_at = <Date 2021-09-08.23:02:31.600>
user = 'https://github.com/hanugit'

bugs.python.org fields:

activity = <Date 2021-09-08.23:02:31.600>
actor = 'hanugit'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = []
creation = <Date 2021-09-08.23:02:31.600>
creator = 'hanugit'
dependencies = []
files = []
hgrepos = []
issue_num = 45143
keywords = []
message_count = 1.0
messages = ['401426']
nosy_count = 1.0
nosy_names = ['hanugit']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue45143'
versions = ['Python 3.9']