pgsql: Doc: fix typo in backup.sgml.

Doc: fix typo in backup.sgml. archive_command was unnecessarily repeated. Author: Tatsuo Ishii Reviewed-by: Amit Kapila Backpatch-through: 15 Discussion: https://postgr.es/m/flat/20230114.110234.666053507266410467.t-ishii%40sranhm.sra.co.jp Branch -- REL_15_STABLE Details --- https://g

pgsql: Doc: fix typo in backup.sgml.

Doc: fix typo in backup.sgml. archive_command was unnecessarily repeated. Author: Tatsuo Ishii Reviewed-by: Amit Kapila Backpatch-through: 15 Discussion: https://postgr.es/m/flat/20230114.110234.666053507266410467.t-ishii%40sranhm.sra.co.jp Branch -- master Details --- https://git.post