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_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/1673a667d4323f056e4cef715cf4c162190ce8e0

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