pgsql: doc: fix plurality typo on bgwriter doc sentence

doc: fix plurality typo on bgwriter doc sentence

Reported-by: matthew.alton@gmail.com

Discussion: /messages/by-id/157204060717.1042.8194076510523669244@wrigleys.postgresql.org

Backpatch-through: 9.4

Branch
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REL9_4_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/00ac1eced6204656ae393a719063fbef99a2cf4b

Modified Files
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doc/src/sgml/bgworker.sgml | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)