doc: reorder attribution of PG 12 btree item

Reported-by: Alexander Korotkov

Discussion: https://postgr.es/m/CAPpHfdvkM-PkyrK6LQitJUDmC_1kOCEtTuseoVhCT=ew0XJmGg@mail.gmail.com

doc/src/sgml/release-12.sgml

@@ -566,7 +566,7 @@ Author: Peter Geoghegan <pg@bowt.ie>
 -->
 
 <para>
-Improve speed of btree index insertions (Alexander Korotkov, Peter Geoghegan)
+Improve speed of btree index insertions (Peter Geoghegan, Alexander Korotkov)
 </para>
 
 <para>