docs: fix broken paths to docs/config/ich9-ehci-uhci.cfg

With the move of some docs/ to docs/devel/ on ac06724a,
a reference path was not updated.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>