pgsql: Fix spi_worker mention in bgworker documentation

Fix spi_worker mention in bgworker documentation

The documentation mentioned contrib/ but the module was moved to
src/test/modules/ by commit 22dfd116a127a of 9.5 era.

Problem pointed out by Dickson Guedes in bug #13896
Backpatch-to: 9.5.

Branch
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REL9_5_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/950beba0035e862e1f884c1222895055dfcf1500

Modified Files
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doc/src/sgml/bgworker.sgml |    3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)