doc: fix capitalization in PG 12 release notes

Reported-by: Thomas Munro

Discussion: https://postgr.es/m/CA+hUKGJpep8uSXoDtVF6iROCRKce-39HEhDPUaYFyMn0U5e9ug@mail.gmail.com

doc/src/sgml/release-12.sgml

@@ -527,7 +527,7 @@ Author: Thomas Munro <tmunro@postgresql.org>
 -->
 
 <para>
-Allow parallel query when in SERIALIZABLE ISOLATION MODE (Thomas Munro)
+Allow parallel query when in SERIALIZABLE isolation mode (Thomas Munro)
 </para>
 
 <para>