relnotes: add Heikki to UTF8 item

Reported-by: John Naylor

Discussion: https://postgr.es/m/CAFBsxsFhbhMbK_ZaLpH6J8BfJL_uowtGg+Qs9XA=F4uPU3aucA@mail.gmail.com

Author: bmomjian

doc/src/sgml/release-15.sgml

@@ -757,7 +757,7 @@ Author: John Naylor <john.naylor@postgresql.org>
 
 <listitem>
 <para>
-Improve validation of UTF-8 text (even if only ASCII) by processing 16 bytes at a time (John Naylor)
+Improve validation of UTF-8 text (even if only ASCII) by processing 16 bytes at a time (John Naylor, Heikki Linnakangas)
 </para>
 
 <para>