pgsql: doc: Fix wording of MERGE actions in README - Mailing list pgsql-committers

doc: Fix wording of MERGE actions in README

UPDATE was listed twice and DELETE was omitted, replace one UPDATE
with DELETE instead.

Backpatch through v15 where MERGE was added.

Author: Myo Wai Thant <myo.waithant@fujitsu.com>
Reviewed-by: Richard Guo <guofenglinux@gmail.com>
Discussion: https://postgr.es/m/OSAPR01MB43247E46931E9E9CFC4AA0F29A079@OSAPR01MB4324.jpnprd01.prod.outlook.com
Backpatch-through: 15

Branch
------
REL_15_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/1eaa48e998e20766d70ed80cf538f3871d1c562c

Modified Files
--------------
src/backend/executor/README | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)