usb: get rid of some ReST doc build errors

We need an space before a numbered list to avoid those warnings:

./drivers/usb/core/message.c:478: ERROR: Unexpected indentation.
./drivers/usb/core/message.c:479: WARNING: Block quote ends without a blank line; unexpected unindent.
./include/linux/usb/composite.h:455: ERROR: Unexpected indentation.
./include/linux/usb/composite.h:456: WARNING: Block quote ends without a blank line; unexpected unindent.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>