[PATCH] fix DocBook build

Fix DocBook build.  Regression was introduced by
gregkh-usb-usb-linux-usb_ch9h-becomes-linux-usb-ch9h.patch

Tested by `make htmldocs`.

Signed-off-by: Don Mullis <dwm@meer.net>
Cc: Greg KH <greg@kroah.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>