greybus: Documentation: Fix N-svc directory name

endo_id was present in place of the directory N-svc, fix it.

Fixes: 4f7b1833e78f ("Documentation/sysfs-bus-greybus: update the bus ABI documentation")
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>