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 Reviewed-by: Richard Guo Discussion: https://postgr.es/m/osapr01mb43247e46931e9e9cfc4aa0f29a..

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 Reviewed-by: Richard Guo Discussion: https://postgr.es/m/osapr01mb43247e46931e9e9cfc4aa0f29a..