pgsql: doc: fix for plurality typo on bgwriter doc sentence

doc: fix for plurality typo on bgwriter doc sentence

Reported-by: Justin Pryzby

Discussion: /messages/by-id/20191106022353.GX4999@telsasoft.com

Backpatch-through: 11, 12

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/4d616b112704411ff059321c76f438691f0f2ae8

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