MINOR: Fix static membership documentation link in upgrade notes (#7001)
MINOR: Fix static membership documentation link in upgrade notes (#7001)

Reviewers: Jason Gustafson <68c46a606457643eab92053c1c05574abb26f861@confluent.io>
guozhangwang/kafka,sslavic/kafka,apache/kafka,guozhangwang/kafka,TiVo/kafka,guozhangwang/kafka,lindong28/kafka,sslavic/kafka,apache/kafka,Chasego/kafka,Chasego/kafka,apache/kafka,noslowerdna/kafka,guozhangwang/kafka,sslavic/kafka,noslowerdna/kafka,TiVo/kafka,TiVo/kafka,lindong28/kafka,apache/kafka,noslowerdna/kafka,Chasego/kafka,TiVo/kafka,Chasego/kafka,sslavic/kafka,lindong28/kafka,lindong28/kafka,noslowerdna/kafka