greybus: Documentation/sysfs: fix sysfs paths

correct sysfs paths typo. (s/device/devices).

Signed-off-by: Sandeep Patil <sspatil@google.com>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>