inetd: Add -S to tftpd usage example in inetd.conf

Since 273a307d0b80743fb08e23237b3f74dc94a8fa2a there is '-S' option which uses
generic file permissions when tftpd is chrooted ('-s /chroot/dir') instead of
special requirements "be publicly writable"

The '-s /tftproot' is always used in the usage example, so add '-S' here too

Reviewed by: eugen
Differential Revision: https://reviews.freebsd.org/D49807