relnotes: add author to in-memory sorts item

Reported-by: Thomas Munro

Discussion: https://postgr.es/m/CA+hUKGKha50-H2ye-9x3PJWU5wBq_MPc2rmVKM1xJ0tL+ntROw@mail.gmail.com

Author: bmomjian

doc/src/sgml/release-15.sgml

@@ -804,7 +804,7 @@ Author: John Naylor <john.naylor@postgresql.org>
 
 <listitem>
 <para>
-Improve performance and reduce memory consumption of in-memory sorts (Ronan Dunklau, David Rowley, Thomas Munro)
+Improve performance and reduce memory consumption of in-memory sorts (Ronan Dunklau, David Rowley, Thomas Munro, John Naylor)
 </para>
 </listitem>