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

Author: danielgustafsson

src/backend/executor/README

@@ -229,7 +229,7 @@ MERGE
 MERGE is a multiple-table, multiple-action command: It specifies a target
 table and a source relation, and can contain multiple WHEN MATCHED and
 WHEN NOT MATCHED clauses, each of which specifies one UPDATE, INSERT,
-UPDATE, or DO NOTHING actions.  The target table is modified by MERGE,
+DELETE, or DO NOTHING actions.  The target table is modified by MERGE,
 and the source relation supplies additional data for the actions.  Each action
 optionally specifies a qualifying expression that is evaluated for each tuple.