Fix spi_worker mention in bgworker documentation

The documentation mentioned contrib/ but the module was moved to
src/test/modules/ by commit 22dfd116 of 9.5 era.

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

doc/src/sgml/bgworker.sgml

@@ -290,7 +290,8 @@ typedef struct BackgroundWorker
   </para>
 
   <para>
-   The <filename>worker_spi</> contrib module contains a working example,
+   The <filename>src/test/modules/worker_spi</> module
+   contains a working example,
    which demonstrates some useful techniques.
   </para>