pgsql: Doc: fix typo in "Generic File Access Functions" section.

Doc: fix typo in "Generic File Access Functions" section. Issue reported by me and fix by Tom Lane. Discussion: https://postgr.es/m/20181219.080458.1434575730369741406.t-ishii%40sraoss.co.jp Branch -- REL_11_STABLE Details --- https://git.postgresql.org/pg/commitdiff/84fcc2ce56962a82e4d

pgsql: Doc: fix typo in "Generic File Access Functions" section.

Doc: fix typo in "Generic File Access Functions" section. Issue reported by me and fix by Tom Lane. Discussion: https://postgr.es/m/20181219.080458.1434575730369741406.t-ishii%40sraoss.co.jp Branch -- master Details --- https://git.postgresql.org/pg/commitdiff/3cab54878dbf212c08699bb974