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.

greenplum-db/gpdb,ashwinstar/gpdb,lisakowen/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,adam8157/gpdb,xinzweb/gpdb,50wu/gpdb,lisakowen/gpdb,adam8157/gpdb,50wu/gpdb,jmcatamney/gpdb,lisakowen/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,lisakowen/gpdb,50wu/gpdb,adam8157/gpdb,adam8157/gpdb,lisakowen/gpdb,xinzweb/gpdb,xinzweb/gpdb,ashwinstar/gpdb,lisakowen/gpdb,ashwinstar/gpdb,jmcatamney/gpdb,adam8157/gpdb,xinzweb/gpdb,ashwinstar/gpdb,50wu/gpdb,50wu/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,xinzweb/gpdb,xinzweb/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,lisakowen/gpdb,greenplum-db/gpdb,lisakowen/gpdb,adam8157/gpdb,50wu/gpdb,greenplum-db/gpdb,50wu/gpdb,adam8157/gpdb,xinzweb/gpdb,xinzweb/gpdb,ashwinstar/gpdb,ashwinstar/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,adam8157/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,jmcatamney/gpdb,50wu/gpdb