doc:  add Heikki to PG 12 release note btree item
doc:  add Heikki to PG 12 release note btree item

Reported-by: Peter Geoghegan

Discussion: e1f3a410ab2e65b4f74f32d70cc6b21e8a76ab35@mail.gmail.com

adam8157/gpdb,50wu/gpdb,greenplum-db/gpdb,adam8157/gpdb,lisakowen/gpdb,greenplum-db/gpdb,xinzweb/gpdb,lisakowen/gpdb,xinzweb/gpdb,adam8157/gpdb,50wu/gpdb,50wu/gpdb,lisakowen/gpdb,lisakowen/gpdb,adam8157/gpdb,50wu/gpdb,greenplum-db/gpdb,adam8157/gpdb,50wu/gpdb,lisakowen/gpdb,lisakowen/gpdb,adam8157/gpdb,50wu/gpdb,greenplum-db/gpdb,xinzweb/gpdb,xinzweb/gpdb,greenplum-db/gpdb,xinzweb/gpdb,adam8157/gpdb,xinzweb/gpdb,xinzweb/gpdb,50wu/gpdb,greenplum-db/gpdb,xinzweb/gpdb,lisakowen/gpdb,lisakowen/gpdb,greenplum-db/gpdb,50wu/gpdb,adam8157/gpdb,greenplum-db/gpdb