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
------
REL_11_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/793739180735a062d9eadec9ad03dbd5db11e5c0

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