Jenkins: add example build ID

wiki/src/contribute/working_together/roles/sysadmins/automated_tests_in_Jenkins.mdwn

@@ -39,7 +39,7 @@ Thankfully, there is a way to trigger a test suite run without having
 to rebuild images first. To do so, start a "build" of the
 corresponding `test_Tail_ISO_*` job, passing to the
 `UPSTREAMJOB_BUILD_NUMBER` parameter the ID of the `build_Tail_ISO_*`
-job build you want to test.
+job build you want to test, for example `10`.
 
 When doing so, you may also pass arbitrary arguments to Cucumber
 via the `EXTRA_CUCUMBER_ARGS` job parameter. For example: