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

docs/usb2.txt

@@ -50,7 +50,7 @@ companion controllers with two ports each.
 There is a config file in docs which will do all this for you, just
 try ...
 
-    qemu -readconfig docs/ich9-ehci-uhci.cfg
+    qemu -readconfig docs/config/ich9-ehci-uhci.cfg
 
 ... then use "bus=ehci.0" to assign your usb devices to that bus.