pgsql: doc: Fix wording of MERGE actions in README

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: /messages/by-id/OSAPR01MB43247E46931E9E9CFC4AA0F29A079@OSAPR01MB4324.jpnprd01.prod.outlook.com
Backpatch-through: 15

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/3b304547a9dfef7101889d35a4c5da7866d66c8e

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