doc: add Heikki to PG 12 release note btree item

Reported-by: Peter Geoghegan

Discussion: https://postgr.es/m/CAH2-WzkrX-aA7d3OYtQT+8Mspq+tU5vwuVz=FTzMH3CdrSyprA@mail.gmail.com

doc/src/sgml/release-12.sgml

@@ -632,7 +632,7 @@ Author: Peter Geoghegan <pg@bowt.ie>
 -->
 
 <para>
-Have new btree indexes sort duplicate index entries in heap-storage order (Peter Geoghegan)
+Have new btree indexes sort duplicate index entries in heap-storage order (Peter Geoghegan, Heikki Linnakangas)
 </para>
 
 <para>