docs: fix broken paths to docs/devel/tracing.txt

With the move of some docs/ to docs/devel/ on ac06724a,
no references were updated.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>