pgsql: doc: fix capitalization in PG 12 release notes

doc: fix capitalization in PG 12 release notes

Reported-by: Thomas Munro

Discussion: /messages/by-id/CA+hUKGJpep8uSXoDtVF6iROCRKce-39HEhDPUaYFyMn0U5e9ug@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/32fe7ee2dd2b2aa8620e69451f60b2b35989677c

Modified Files
--------------
doc/src/sgml/release-12.sgml | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)